environment’s outer environment reference is null. A global environment’s EnvironmentRecord may be
prepopulated with identifier bindings and includes an associated global object whose properties provide some of the global environment’s identifier bindings. This global object is the
value of a global environment’s this
binding. As ECMAScript code is executed, additional properties may
be added to the global object and the initial properties may be modified.
A module environment is a Lexical Environment that contains the bindings for the top level declarations of a Module. It also contains the bindings that are explicitly imported by the Module. The outer environment of a module environment is a global environment.
A function environment is a Lexical Environment that corresponds to the invocation of an ECMAScript function object. A function environment may establish a new
this
binding. A function environment also captures the state necessary to support super
method
invocations.
Lexical Environments and Environment Record values are purely specification mechanisms and need not correspond to any specific artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to directly access or manipulate such values.
There are two primary kinds of Environment Record values used in this specification: declarative Environment Records and object Environment Records. Declarative Environment Records are used to define the effect of ECMAScript language syntactic elements such as FunctionDeclarations, VariableDeclarations, and Catch clauses that directly associate identifier bindings with ECMAScript language values. Object Environment Records are used to define the effect of ECMAScript elements such as WithStatement that associate identifier bindings with the properties of some object. Global Environment Records and function Environment Records are specializations that are used for specifically for Script global declarations and for top-level declarations within functions.
For specification purposes Environment Record values are values of the Record specification type and can be thought of as existing in a simple object-oriented hierarchy where Environment Record is an abstract class with three concrete subclasses, declarative Environment Record, object Environment Record, and global Environment Record. Function Environment Records and module Environment Records are subclasses of declarative Environment Record. The abstract class includes the abstract specification methods defined in Table 15. These abstract methods have distinct concrete algorithms for each of the concrete subclasses.
| Method | Purpose |
|---|---|
| HasBinding(N) | Determine if an Environment Record has a binding for the String value N. Return true if it does and false if it does not |
| CreateMutableBinding(N, D) | Create a new but uninitialized mutable binding in an Environment Record. The String value N is the text of the bound name. If the optional Boolean argument D is true the binding is may be subsequently deleted. |
| CreateImmutableBinding(N, S) | Create a new but uninitialized immutable binding in an Environment Record. The String value N is the text of the bound name. If S is true then attempts to access the value of the binding before it is initialized or set it after it has been initialized will always throw an exception, regardless of the strict mode setting of operations that reference that binding. S is an optional parameter that defaults to false. |
| InitializeBinding(N,V) | Set the value of an already existing but uninitialized binding in an Environment Record. The String value N is the text of the bound name. V is the value for the binding and is a value of any ECMAScript language type. |
| SetMutableBinding(N,V, S) | Set the value of an already existing mutable binding in an Environment Record. The String value N is the text of the bound name. V is the value for the binding and may be a value of any ECMAScript language type. S is a Boolean flag. If S is true and the binding cannot be set throw a TypeError exception. |
| GetBindingValue(N,S) | Returns the value of an already existing binding from an Environment Record. The String value N is the text of the bound name. S is used to identify references originating in strict mode code or that otherwise require strict mode reference semantics. If S is true and the binding does not exist throw a ReferenceError exception. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of S. |
| DeleteBinding(N) | Delete a binding from an Environment Record. The String value N is the text of the bound name. If a binding for N exists, remove the binding and return true. If the binding exists but cannot be removed return false. If the binding does not exist return true. |
| HasThisBinding() | Determine if an Environment Record establishes a this binding. Return true if it does and false if it does not. |
| HasSuperBinding() | Determine if an Environment Record establishes a super method binding. Return true if it does and false if it does not. |
| WithBaseObject () | If this Environment Record is associated with a with statement, return the with object. Otherwise, return undefined. |
Each declarative Environment Record is associated with an ECMAScript program scope containing variable, constant, let, class, module, import, and/or function declarations. A declarative Environment Record binds the set of identifiers defined by the declarations contained within its scope.
The behaviour of the concrete specification methods for declarative Environment Records is defined by the following algorithms.
The concrete Environment Record method HasBinding for declarative Environment Records simply determines if the argument identifier is one of the identifiers bound by the record:
The concrete Environment Record method CreateMutableBinding for declarative Environment Records creates a new mutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If Boolean argument D is provided and has the value true the new binding is marked as being subject to deletion.
The concrete Environment Record method CreateImmutableBinding for declarative Environment Records creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If Boolean argument S is provided and has the value true the new binding is marked as a strict binding.
The concrete Environment Record method InitializeBinding for declarative Environment Records is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist.
The concrete Environment Record method SetMutableBinding for declarative Environment Records attempts to change the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. A binding for N normally already exist, but in rare cases it may not. If the binding is an immutable binding, a TypeError is thrown if S is true.
NOTE An example of ECMAScript code that results in a missing binding at step 2 is:
function f(){eval("var x; x = (delete x, 0);")}
The concrete Environment Record method GetBindingValue for declarative Environment Records simply returns the value of its bound identifier whose name is the value of the argument N. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of S.
The concrete Environment Record method DeleteBinding for declarative Environment Records can only delete bindings that have been explicitly designated as being subject to deletion.
Regular declarative Environment Records do not provide a this
binding.
Regular declarative Environment Records do not provide a super
binding.
Declarative Environment Records always return undefined as their WithBaseObject.
Each object Environment Record is associated with an object called its binding object. An object Environment Record binds the set of string identifier names that directly correspond to the property names of its binding object. Property keys that are not strings in the form of an IdentifierName are not included in the set of bound identifiers. Both own and inherited properties are included in the set regardless of the setting of their [[Enumerable]] attribute. Because properties can be dynamically added and deleted from objects, the set of identifiers bound by an object Environment Record may potentially change as a side-effect of any operation that adds or deletes properties. Any bindings that are created as a result of such a side-effect are considered to be a mutable binding even if the Writable attribute of the corresponding property has the value false. Immutable bindings do not exist for object Environment Records.
Object Environment Records created for with
statements (13.11) can
provide their binding object as an implicit this value for use in function calls. The capability is controlled by
a withEnvironment Boolean value that is associated with each object Environment Record. By default, the value of withEnvironment is
false for any object Environment Record.
The behaviour of the concrete specification methods for object Environment Records is defined by the following algorithms.
The concrete Environment Record method HasBinding for object Environment Records determines if its associated binding object has a property whose name is the value of the argument N:
The concrete Environment Record method CreateMutableBinding for object Environment Records creates in an Environment Record’s associated binding object a property whose name is the String value and initializes it to the value undefined. If Boolean argument D is provided and has the value true the new property’s [[Configurable]] attribute is set to true, otherwise it is set to false.
NOTE Normally envRec will not have a binding for N but if it does, the semantics of DefinePropertyOrThrow may result in an existing binding being replaced or shadowed or cause an abrupt completion to be returned.
The concrete Environment Record method CreateImmutableBinding is never used within this specification in association with Object Environment Records.
The concrete Environment Record method InitializeBinding for object Environment Records is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist.
NOTE In this specification, all uses of CreateMutableBinding for object Environment Records are immediately followed by a call to InitializeBinding for the same name. Hence, implementations do not need to explicitly track the initialization state of individual object Environment Record bindings.
The concrete Environment Record method SetMutableBinding for object Environment Records attempts to set the value of the Environment Record’s associated binding object’s property whose name is the value of the argument N to the value of argument V. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument S.
The concrete Environment Record method GetBindingValue for object Environment Records returns the value of its associated binding object’s property whose name is the String value of the argument identifier N. The property should already exist but if it does not the result depends upon the value of the S argument:
The concrete Environment Record method DeleteBinding for object Environment Records can only delete bindings that correspond to properties of the environment object whose [[Configurable]] attribute have the value true.
Regular object Environment Records do not provide a this
binding.
Regular object Environment Records do not provide a super
binding.
Object Environment Records return undefined as their WithBaseObject unless their withEnvironment flag is true.
A function Environment Record is a declarative Environment Record that is used to represent the top-level scope of a function and,
if the function is not an ArrowFunction, provides a this
binding. If a function is
not an ArrowFunction function and references super
, its function Environment Record also contains the state that is used to perform
super
method invocations from within the function.
Function Environment Records have the additional state fields listed in Table 16.
| Field Name | Value | Meaning |
|---|---|---|
| [[thisValue]] | Any | This is the this value used for this invocation of the function. |
| [[thisBindingStatus]] | "lexical" | "initialized" | "uninitialized" |
If the value is "lexical" , this is an ArrowFunction and does not have a local this value. |
| [[FunctionObject]] | Object | The function Object whose invocation caused this Environment Record to be created. |
| [[HomeObject]] | Object | undefined | If the associated function has super property accesses and is not an ArrowFunction, [[HomeObject]] is the object that the function is bound to as a method. The default value for [[HomeObject]] is undefined. |
| [[NewTarget]] | Object | undefined | If this Environment Record was created by the [[Construct]] internal method, [[NewTarget]] is the value of the [[Construct]] newTarget parameter. Otherwise, its value is undefined. |
Function Environment Records support all of the declarative Environment Record methods listed in Table 15 and share the same specifications for all of those methods except for HasThisBinding and HasSuperBinding. In addition, function Environment Records support the methods listed in Table 17:
| Method | Purpose |
|---|---|
| BindThisValue(V) | Set the [[thisValue]] and record that it has been initialized. |
| GetThisBinding() | Return the value of this Environment Record’s this binding. Throws a ReferenceError if the this binding has not been initialized. |
| GetSuperBase() | Return the object that is the base for super property accesses bound in this Environment Record. The object is derived from this Environment Record’s [[HomeObject]] field. The value undefined indicates that super property accesses will produce runtime errors. |
The behaviour of the additional concrete specification methods for function Environment Records is defined by the following algorithms:
"lexical"
."initialized"
, throw a ReferenceError
exception."initialized"
."lexical"
, return false; otherwise, return
true."lexical"
, return false."lexical"
."uninitialized"
, throw a ReferenceError
exception.A global Environment Record is used to represent the outer most scope that is shared by all of the ECMAScript Script elements that are processed in a common Realm (8.2). A global Environment Record provides the bindings for built-in globals (clause 18), properties of the global object, and for all top-level declarations (13.2.8, 13.2.10) that occur within a Script.
A global Environment Record is logically a single record but it is specified as a composite encapsulating an object Environment Record and a declarative Environment Record. The object Environment Record has as its base object the global object of the associated Realm. This global object is the value returned by the global Environment Record’s GetThisBinding concrete method. The object Environment Record component of a global Environment Record contains the bindings for all built-in globals (clause 18) and all bindings introduced by a FunctionDeclaration, GeneratorDeclaration, or VariableStatement contained in global code. The bindings for all other ECMAScript declarations in global code are contained in the declarative Environment Record component of the global Environment Record.
Properties may be created directly on a global object. Hence, the object Environment Record component of a global Environment Record may contain both bindings created explicitly by FunctionDeclaration, GeneratorDeclaration, or VariableDeclaration declarations and binding created implicitly as properties of the global object. In order to identify which bindings were explicitly created using declarations, a global Environment Record maintains a list of the names bound using its CreateGlobalVarBindings and CreateGlobalFunctionBindings concrete methods.
Global Environment Records have the additional fields listed in Table 18 and the additional methods listed in Table 19.
| Field Name | Value | Meaning |
|---|---|---|
| [[ObjectRecord]] | Object Environment Record | Binding object is the global object. It contains global built-in bindings as well as FunctionDeclaration, GeneratorDeclaration, and VariableDeclaration bindings in global code for the associated Realm. |
| [[DeclarativeRecord]] | Declarative Environment Record | Contains bindings for all declarations in global code for the associated Realm code except for FunctionDeclaration, GeneratorDeclaration, and VariableDeclaration bindings. |
| [[VarNames]] | List of String | The string names bound by FunctionDeclaration, GeneratorDeclaration, and VariableDeclaration declarations in global code for the associated Realm. |
| Method | Purpose |
|---|---|
| GetThisBinding() | Return the value of this Environment Record’s this binding. |
| HasVarDeclaration (N) | Determines if the argument identifier has a binding in this Environment Record that was created using a VariableDeclaration, FunctionDeclaration, or GeneratorDeclaration. |
| HasLexicalDeclaration (N) | Determines if the argument identifier has a binding in this Environment Record that was created using a lexical declaration such as a LexicalDeclaration or a ClassDeclaration. |
| HasRestrictedGlobalProperty (N) | Determines if the argument is the name of a global object property that may not be shadowed by a global lexically binding. |
| CanDeclareGlobalVar (N) | Determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N. |
| CanDeclareGlobalFunction (N) | Determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N. |
| CreateGlobalVarBinding(N, D) | Used to create and initialize to undefined a global var binding in the [[ObjectRecord]] component of a global Environment Record. The binding will be a mutable binding. The corresponding global object property will have attribute values appropriate for a var . The String value N is the bound name. If D is true the binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows var declarations to receive special treatment. |
| CreateGlobalFunctionBinding(N, V, D) | Create and initialize a global function binding in the [[ObjectRecord]] component of a global Environment Record. The binding will be a mutable binding. The corresponding global object property will have attribute values appropriate for a function . The String value N is the bound name. V is the initialization value. If the optional Boolean argument D is true the binding is may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows function declarations to receive special treatment. |
The behaviour of the concrete specification methods for global Environment Records is defined by the following algorithms.
The concrete Environment Record method HasBinding for global Environment Records simply determines if the argument identifier is one of the identifiers bound by the record:
The concrete Environment Record method CreateMutableBinding for global Environment Records creates a new mutable binding for the name N that is uninitialized. The binding is created in the associated DeclarativeRecord. A binding for N must not already exist in the DeclarativeRecord. If Boolean argument D is provided and has the value true the new binding is marked as being subject to deletion.
The concrete Environment Record method CreateImmutableBinding for global Environment Records creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If Boolean argument S is provided and has the value true the new binding is marked as a strict binding.
The concrete Environment Record method InitializeBinding for global Environment Records is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist.
The concrete Environment Record method SetMutableBinding for global Environment Records attempts to change the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. If the binding is an immutable binding, a TypeError is thrown if S is true. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument S.
The concrete Environment Record method GetBindingValue for global Environment Records returns the value of its bound identifier whose name is the value of the argument N. If the binding is an uninitialized binding throw a ReferenceError exception. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument S.
The concrete Environment Record method DeleteBinding for global Environment Records can only delete bindings that have been explicitly designated as being subject to deletion.
Global Environment Records always provide a
this
binding whose value is the associated global object.
Global Environment Records always return undefined as their WithBaseObject.
The concrete Environment Record method HasVarDeclaration for global Environment Records determines if the argument identifier has a binding in this record that was created using a VariableStatement or a FunctionDeclaration :
The concrete Environment Record method HasLexicalDeclaration for global Environment Records determines if the argument identifier has a binding in this record that was created using a lexical declaration such as a LexicalDeclaration or a ClassDeclaration :
The concrete Environment Record method HasRestrictedGlobalProperty for global Environment Records determines if the argument identifier is the name of a property of the global object that must not be shadowed by a global lexically binding:
NOTE Properties may exist upon a global object that were directly created rather than being
declared using a var or function declaration. A global lexical binding may not be created that has the same name as a
non-configurable property of the global object. The global property undefined
is an example of such a
property.
The concrete Environment Record method CanDeclareGlobalVar for global Environment Records determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N. Redundant var declarations and var declarations for pre-existing global object properties are allowed.
The concrete Environment Record method CanDeclareGlobalFunction for global Environment Records determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N.
The concrete Environment Record method CreateGlobalVarBinding for global Environment Records creates and initializes a mutable binding in the associated object Environment Record and records the bound name in the associated [[VarNames]] List. If a binding already exists, it is reused and assumed to be initialized.
The concrete Environment Record method CreateGlobalFunctionBinding for global Environment Records creates and initializes a mutable binding in the associated object Environment Record and records the bound name in the associated [[VarNames]] List. If a binding already exists, it is replaced.
NOTE Global function declarations are always represented as own properties of the global object. If possible, an existing own property is reconfigured to have a standard set of attribute values. Steps 10-12 are equivalent to what calling the InitializeBinding concrete method would do and if globalObject is a Proxy will produce the same sequence of Proxy trap calls.
A module Environment Record is a declarative Environment Record that is used to represent the outer scope of an ECMAScript Module. In additional to normal mutable and immutable bindings, module Environment Records also provide immutable import bindings which are bindings that provide indirect access to a target binding that exists in another Environment Record.
Module Environment Records support all of the declarative Environment Record methods listed in Table 15 and share the same specifications for all of those methods except for GetBindingValue, DeleteBinding, HasThisBinding and GetThisBinding. In addition, module Environment Records support the methods listed in Table 20:
| Method | Purpose |
|---|---|
| CreateImportBinding(N, M, N2 ) | Create an immutable indirect binding in a module Environment Record. The String value N is the text of the bound name. M is a Module Record (see <IP_ADDRESS>), and N2 is a binding that exists in M’s module Environment Record. |
| GetThisBinding() | Return the value of this Environment Record’s this binding. |
The behaviour of the additional concrete specification methods for module Environment Records are defined by the following algorithms:
The concrete Environment Record method GetBindingValue for module Environment Records returns the value of its bound identifier whose name is the value of the argument N. However, if the binding is an indirect binding the value of the target binding is returned. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of S.
NOTE Because a Module is always strict mode code, calls to GetBindingValue should always pass true as the value of S.
The concrete Environment Record method DeleteBinding for module Environment Records refuses to delete bindings.
NOTE The bindings of a module Environment Record are not deletable.
Module Environment Records provide a this
binding.
The concrete Environment Record method CreateImportBinding for module Environment Records creates a new initialized immutable indirect binding for the name N. A binding must not already exist in this Environment Record for N. M is a Module Record (see <IP_ADDRESS>), and N2 is the name of a binding that exists in M’s module Environment Record. Accesses to the value of the new binding will indirectly access the bound value of value of the target binding.
The following abstract operations are used in this specification to operate upon lexical environments:
The abstract operation GetIdentifierReference is called with a Lexical Environment lex, a String name, and a Boolean flag strict. The value of lex may be null. When called, the following steps are performed:
When the abstract operation NewDeclarativeEnvironment is called with a Lexical Environment as argument E the following steps are performed:
When the abstract operation NewObjectEnvironment is called with an Object O and a Lexical Environment E as arguments, the following steps are performed:
When the abstract operation NewFunctionEnvironment is called with arguments F and newTarget the following steps are performed:
"lexical"
."uninitialized"
.When the abstract operation NewGlobalEnvironment is called with an ECMAScript Object G as its argument, the following steps are performed:
When the abstract operation NewModuleEnvironment is called with a Lexical Environment argument E the following steps are performed:
Before it is evaluated, all ECMAScript code must be associated with a Realm. Conceptually, a realm consists of a set of intrinsic objects, an ECMAScript global environment, all of the ECMAScript code that is loaded within the scope of that global environment, and other associated state and resources.
A Realm is specified as a Record with the fields specified in Table 21:
| Field Name | Value | Meaning |
|---|---|---|
| [[intrinsics]] | Record whose field names are intrinsic keys and whose values are objects | These are the intrinsic values used by code associated with this Realm |
| [[globalThis]] | Object | The global object for this Realm |
| [[globalEnv]] | Lexical Environment | The global environment for this Realm |
| [[templateMap]] | A List of Record { [[strings]]: List, [[array]]: Object}. | Template objects are canonicalized separately for each Realm using its [[templateMap]]. Each [[strings]] value is a List containing, in source text order, the raw String values of a TemplateLiteral that has been evaluated. The associated [[array]] value is the corresponding template object that is passed to a tag function. |
An implementation may define other, implementation specific fields.
The abstract operation CreateRealm with no arguments performs the following steps:
When the abstract operation CreateIntrinsics with argument realmRec performs the following steps:
The abstract operation SetRealmGlobalObject with arguments realmRec and globalObj performs the following steps:
The abstract operation SetDefaultGlobalBindings with argument realmRec performs the following steps:
An execution context is a specification device that is used to track the runtime evaluation of code by an ECMAScript implementation. At any point in time, there is at most one execution context that is actually executing code. This is known as the running execution context. A stack is used to track execution contexts. The running execution context is always the top element of this stack. A new execution context is created whenever control is transferred from the executable code associated with the currently running execution context to executable code that is not associated with that execution context. The newly created execution context is pushed onto the stack and becomes the running execution context.
An execution context contains whatever implementation specific state is necessary to track the execution progress of its associated code. Each execution context has at least the state components listed in Table 22.
| Component | Purpose |
|---|---|
| code evaluation state | Any state needed to perform, suspend, and resume evaluation of the code associated with this execution context. |
| Function | If this execution context is evaluating the code of a function object, then the value of this component is that function object. If the context is evaluating the code of a Script or Module, the value is null. |
| Realm | The Realm from which associated code accesses ECMAScript resources. |
Evaluation of code by the running execution context may be suspended at various points defined within this specification. Once the running execution context has been suspended a different execution context may become the running execution context and commence evaluating its code. At some later time a suspended execution context may again become the running execution context and continue evaluating its code at the point where it had previously been suspended. Transition of the running execution context status among execution contexts usually occurs in stack-like last-in/first-out manner. However, some ECMAScript features require non-LIFO transitions of the running execution context.
The value of the Realm component of the running execution context is also called the current Realm. The value of the Function component of the running execution context is also called the active function object.
Execution contexts for ECMAScript code have the additional state components listed in Table 23.
| Component | Purpose |
|---|---|
| LexicalEnvironment | Identifies the Lexical Environment used to resolve identifier references made by code within this execution context. |
| VariableEnvironment | Identifies the Lexical Environment whose EnvironmentRecord holds bindings created by VariableStatements within this execution context. |
The LexicalEnvironment and VariableEnvironment components of an execution context are always Lexical Environments. When an execution context is created its LexicalEnvironment and VariableEnvironment components initially have the same value.
Execution contexts representing the evaluation of generator objects have the additional state components listed in Table 24.
| Component | Purpose |
|---|---|
| Generator | The GeneratorObject that this execution context is evaluating. |
In most situations only the running execution context (the top of the execution context stack) is directly manipulated by algorithms within this specification. Hence when the terms “LexicalEnvironment”, and “VariableEnvironment” are used without qualification they are in reference to those components of the running execution context.
An execution context is purely a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation. It is impossible for ECMAScript code to directly access or observe an execution context.
The ResolveBinding abstract operation is used to determine the binding of name passed as a String value. The optional argument env can be used to explicitly provide the Lexical Environment that is to be searched for the binding. During execution of ECMAScript code, ResolveBinding is performed using the following algorithm:
NOTE The result of ResolveBinding is always a Reference value with its referenced name component equal to the name argument.
The abstract operation GetThisEnvironment finds
the Environment Record that currently supplies the binding of the keyword
this
. GetThisEnvironment performs the following steps:
NOTE The loop in step 2 will always terminate because the list of environments always ends with
the global environment which has a this
binding.
The abstract operation ResolveThisBinding determines the binding of the keyword this
using the LexicalEnvironment of the running
execution context. ResolveThisBinding performs the following steps:
The abstract operation GetNewTarget determines the NewTarget value using the LexicalEnvironment of the running execution context. GetNewTarget performs the following steps:
The abstract operation GetGlobalObject returns the global object used by the currently running execution context. GetGlobalObject performs the following steps:
A Job is an abstract operation that initiates an ECMAScript computation when no other ECMAScript computation is currently in progress. A Job abstract operation may be defined to accept an arbitrary set of job parameters.
Execution of a Job can be initiated only when there is no running execution context and the execution context stack is empty. A PendingJob is a request for the future execution of a Job. A PendingJob is an internal Record whose fields are specified in Table 25. Once execution of a Job is initiated, the Job always executes to completion. No other Job may be initiated until the currently running Job completes. However, the currently running Job or external events may cause the enqueuing of additional PendingJobs that may be initiated sometime after completion of the currently running Job.
| Field Name | Value | Meaning |
|---|---|---|
| [[Job]] | The name of a Job abstract operation | This is the abstract operation that is performed when execution of this PendingJob is initiated. Jobs are abstract operations that use NextJob rather than Return to indicate that they have completed. |
| [[Arguments]] | A List | The List of argument values that are to be passed to [[Job]] when it is activated. |
| [[Realm]] | A Realm Record | The Realm for the initial execution context when this Pending Job is initiated. |
| [[HostDefined]] | Any, default value is undefined. | Field reserved for use by host environments that need to associate additional information with a pending Job. |
A Job Queue is a FIFO queue of PendingJob records. Each Job Queue has a name and the full set of available Job Queues are defined by an ECMAScript implementation. Every ECMAScript implementation has at least the Job Queues defined in Table 26.
| Name | Purpose |
|---|---|
| ScriptJobs | Jobs that validate and evaluate ECMAScript Script and Module source text. See clauses 10 and 15. |
| PromiseJobs | Jobs that are responses to the settlement of a Promise (see 25.4). |
A request for the future execution of a Job is made by enqueueing, on a Job Queue, a PendingJob record that includes a Job abstract operation name and any necessary argument values. When there is no running execution context and the execution context stack is empty, the ECMAScript implementation removes the first PendingJob from a Job Queue and uses the information contained in it to create an execution context and starts execution of the associated Job abstract operation.
The PendingJob records from a single Job Queue are always initiated in FIFO order. This specification does not define the order in which multiple Job Queues are serviced. An ECMAScript implementation may interweave the FIFO evaluation of the PendingJob records of a Job Queue with the evaluation of the PendingJob records of one or more other Job Queues. An implementation must define what occurs when there are no running execution context and all Job Queues are empty.
NOTE Typically an ECMAScript implementation will have its Job Queues pre-initialized with at least one PendingJob and one of those Jobs will be the first to be executed. An implementation might choose to free all resources and terminate if the current Job completes and all Job Queues are empty. Alternatively, it might choose to wait for a some implementation specific agent or mechanism to enqueue new PendingJob requests.
The following abstract operations are used to create and manage Jobs and Job Queues:
The EnqueueJob abstract operation requires three arguments: queueName, job, and arguments. It performs the following steps:
An algorithm step such as:
is used in Job abstract operations in place of:
Job abstract operations must not contain a Return step or a ReturnIfAbrupt step. The NextJob result operation is equivalent to the following steps:
An ECMAScript implementation performs the following steps prior to the execution of any Jobs or the evaluation of any ECMAScript code:
"ScriptJobs"
, ScriptEvaluationJob, « sourceText »)."ScriptJobs"
, TopLevelModuleEvaluationJob, « sourceText
»).The abstract operation InitializeHostDefinedRealm with parameter realm performs the following steps:
All ordinary objects have an internal slot called [[Prototype]]. The value of this internal slot is either null or an object and is used for implementing inheritance. Data properties of the [[Prototype]] object are inherited (are visible as properties of the child object) for the purposes of get access, but not for set access. Accessor properties are inherited for both get access and set access.
Every ordinary object has a Boolean-valued [[Extensible]] internal slot that controls whether or not properties may be added to the object. If the value of the [[Extensible]] internal slot is false then additional properties may not be added to the object. In addition, if [[Extensible]] is false the value of the [[Prototype]] internal slot of the object may not be modified. Once the value of an object’s [[Extensible]] internal slot has been set to false it may not be subsequently changed to true.
In the following algorithm descriptions, assume O is an ordinary object, P is a property key value, V is any ECMAScript language value, and Desc is a Property Descriptor record.
When the [[GetPrototypeOf]] internal method of O is called the following steps are taken:
When the [[SetPrototypeOf]] internal method of O is called with argument V the following steps are taken:
NOTE The loop in step 8 guarantees that there will be no circularities in any prototype chain that only includes objects that use the ordinary object definitions for [[GetPrototypeOf]] and [[SetPrototypeOf]].
When the [[IsExtensible]] internal method of O is called the following steps are taken:
When the [[PreventExtensions]] internal method of O is called the following steps are taken:
When the [[GetOwnProperty]] internal method of O is called with property key P, the following steps are taken:
When the abstract operation OrdinaryGetOwnProperty is called with Object O and with property key P, the following steps are taken:
When the [[DefineOwnProperty]] internal method of O is called with property key P and Property Descriptor Desc, the following steps are taken:
When the abstract operation OrdinaryDefineOwnProperty is called with Object O, property key P, and Property Descriptor Desc the following steps are taken:
When the abstract operation IsCompatiblePropertyDescriptor is called with Boolean value Extensible, and Property Descriptors Desc, and Current the following steps are taken:
When the abstract operation ValidateAndApplyPropertyDescriptor is called with Object O, property key P, Boolean value extensible, and Property Descriptors Desc, and current the following steps are taken:
This algorithm contains steps that test various fields of the Property Descriptor Desc for specific values. The fields that are tested in this manner need not actually exist in Desc. If a field is absent then its value is considered to be false.
NOTE 1 If undefined is passed as the O argument only validation is performed and no object updates are performed.
NOTE 2 Step 8.b allows any field of Desc to be different from the corresponding field of current if current’s [[Configurable]] field is true. This even permits changing the [[Value]] of a property whose [[Writable]] attribute is false. This is allowed because a true [[Configurable]] attribute would permit an equivalent sequence of calls where [[Writable]] is first set to true, a new [[Value]] is set, and then [[Writable]] is set to false.
When the [[HasProperty]] internal method of O is called with property key P, the following steps are taken:
When the abstract operation OrdinaryHasProperty is called with Object O and with property key P, the following steps are taken:
When the [[Get]] internal method of O is called with property key P and ECMAScript language value Receiver the following steps are taken:
When the [[Set]] internal method of O is called with property key P, value V, and ECMAScript language value Receiver, the following steps are taken:
When the [[Delete]] internal method of O is called with property key P the following steps are taken:
When the [[Enumerate]] internal method of O is called the following steps are taken:
next
method iterates
over all the String-valued keys of enumerable properties of O. The Iterator object must inherit from
%IteratorPrototype% (25.1.2). The mechanics and order of enumerating the
properties is not specified but must conform to the rules specified below.The iterator’s next
method processes object properties to determine whether the property key should be returned as an iterator value. Returned property keys do not include keys
that are Symbols. Properties of the target object may be deleted during enumeration. A property that is deleted before it is
processed by the iterator’s next
method is ignored. If new properties are added to the target object
during enumeration, the newly added properties are not guaranteed to be processed in the active enumeration. A property name
will be returned by the iterator’s next
method at most once in any enumeration.
Enumerating the properties of the target object includes enumerating properties of its prototype, and the prototype of
the prototype, and so on, recursively; but a property of a prototype is not processed if it has the same name as a property
that has already been processed by the iterator’s next
method. The values of [[Enumerable]] attributes
are not considered when determining if a property of a prototype object has already been processed. The enumerable property
names of prototype objects must be obtained as if by invoking the prototype object’s [[Enumerate]] internal method.
[[Enumerate]] must obtain the own property keys of the target object as if by calling its [[OwnPropertyKeys]] internal
method. Property attributes of the target object must be obtained as if by calling its [[GetOwnProperty]] internal
method.
NOTE The following is an informative definition of an ECMAScript generator function that conforms to these rules:
function* enumerate(obj) {
let visited=new Set;
for (let key of Reflect.ownKeys(obj)) {
if (typeof key === "string") {
let desc = Reflect.getOwnPropertyDescriptor(obj,key);
if (desc) {
visited.add(key);
if (desc.enumerable) yield key;
}
}
}
let proto = Reflect.getPrototypeOf(obj)
if (proto === null) return;
for (let protoName of Reflect.enumerate(proto)) {
if (!visited.has(protoName)) yield protoName;
}
}
When the [[OwnPropertyKeys]] internal method of O is called the following steps are taken:
The abstract operation ObjectCreate with argument proto (an object or null) is used to specify the runtime creation of new ordinary objects. The optional argument internalSlotsList is a List of the names of additional internal slots that must be defined as part of the object. If the list is not provided, an empty List is used. This abstract operation performs the following steps:
The abstract operation OrdinaryCreateFromConstructor creates an ordinary object whose [[Prototype]]
value is retrieved from a constructor’s prototype
property, if it exists. Otherwise the intrinsic named
by intrinsicDefaultProto is used for [[Prototype]]. The optional internalSlotsList is a List of the names of additional internal slots that must be defined as
part of the object. If the list is not provided, an empty List is
used. This abstract operation performs the following steps:
The abstract operation GetPrototypeFromConstructor determines the [[Prototype]] value that should be
used to create an object corresponding to a specific constructor. The value is retrieved from the constructor’s
prototype
property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for
[[Prototype]]. This abstract operation performs the following steps:
"prototype"
).NOTE If constructor does not supply a [[Prototype]] value, the default value that is used is obtained from the Code Realm of the constructor function rather than from the running execution context.
ECMAScript function objects encapsulate parameterized ECMAScript code closed over a lexical environment and support the dynamic evaluation of that code. An ECMAScript function object is an ordinary object and has the same internal slots and the same internal methods as other ordinary objects. The code of an ECMAScript function object may be either strict mode code (10.2.1) or non-strict mode code. An ECMAScript function object whose code is strict mode code is called a strict function. One whose code is not strict mode code is called a non-strict function.
ECMAScript function objects have the additional internal slots listed in Table 27.
| Internal Slot | Type | Description |
|---|---|---|
| [[Environment]] | Lexical Environment | The Lexical Environment that the function was closed over. Used as the outer environment when evaluating the code of the function. |
| [[FormalParameters]] | Parse Node | The root parse node of the source text that defines the function’s formal parameter list. |
| [[FunctionKind]] | String | Either "normal" , "classConstructor" or "generator" . |
| [[ECMAScriptCode]] | Parse Node | The root parse node of the source text that defines the function’s body. |
| [[ConstructorKind]] | String | Either "base" or "derived" . |
| [[Realm]] | Realm Record | The Code Realm in which the function was created and which provides any intrinsic objects that are accessed when evaluating the function. |
| [[ThisMode]] | (lexical, strict, global) | Defines how this references are interpreted within the formal parameters and code body of the function. lexical means that this refers to the this value of a lexically enclosing function. strict means that the this value is used exactly as provided by an invocation of the function. global means that a this value of undefined is interpreted as a reference to the global object. |
| [[Strict]] | Boolean | true if this is a strict mode function, false if this is not a strict mode function. |
| [[HomeObject]] | Object | If the function uses super , this is the object whose [[GetPrototypeOf]] provides the object where super property lookups begin. |
All ECMAScript function objects have the [[Call]] internal method defined here. ECMAScript functions that are also constructors in addition have the [[Construct]] internal method. ECMAScript function objects whose code is not strict mode code have the [[GetOwnProperty]] internal method defined here.
The [[Call]] internal method for an ECMAScript function object F is called with parameters thisArgument and argumentsList, a List of ECMAScript language values. The following steps are taken:
"classConstructor"
, throw a TypeError exception.NOTE When calleeContext is removed from the execution context stack in step 8 it must not be destroyed if it is suspended and retained for later resumption by an accessible generator object.
When the abstract operation PrepareForOrdinaryCall is called with function object F and ECMAScript language value newTarget, the following steps are taken:
When the abstract operation OrdinaryCallBindThis is called with function object F, execution context calleeContext, and ECMAScript value thisArgument the following steps are taken:
"uninitialized"
.When the abstract operation OrdinaryCallEvaluateBody is called with function object F and List argumentsList the following steps are taken:
The [[Construct]] internal method for an ECMAScript Function object F is called with parameters argumentsList and newTarget. argumentsList is a possibly empty List of ECMAScript language values. The following steps are taken:
"base"
, then
"%ObjectPrototype%"
)."base"
, perform OrdinaryCallBindThis(F,
calleeContext, thisArgument)."base"
, return NormalCompletion(thisArgument).The abstract operation FunctionAllocate requires the two arguments functionPrototype and strict. It also accepts one optional argument, functionKind. FunctionAllocate performs the following steps:
"normal"
, "non-constructor"
or "generator"
."normal"
."non-constructor"
, then
"normal"
."generator"
, set the [[ConstructorKind]] internal slot of F to
"derived"
."base"
."derived"
constructors to prevent [[Construct]] from
preallocating a generator instance. Generator instance objects are allocated when EvaluateBody is applied to the
GeneratorBody of a generator function.The abstract operation FunctionInitialize requires the arguments: a function object F, kind which is one of (Normal, Method, Arrow), a parameter list production specified by ParameterList, a body production specified by Body, a Lexical Environment specified by Scope. FunctionInitialize performs the following steps:
length
own property."length"
,
PropertyDescriptor{[[Value]]: len, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true}).The abstract operation FunctionCreate requires the arguments: kind which is one of (Normal, Method, Arrow), a parameter list production specified by ParameterList, a body production specified by Body, a Lexical Environment specified by Scope, a Boolean flag Strict, and optionally, an object prototype. FunctionCreate performs the following steps:
"non-constructor"
."normal"
.The abstract operation GeneratorFunctionCreate requires the arguments: kind which is one of (Normal, Method), a parameter list production specified by ParameterList, a body production specified by Body, a Lexical Environment specified by Scope, and a Boolean flag Strict. GeneratorFunctionCreate performs the following steps:
"generator"
).The abstract operation AddRestrictedFunctionProperties is called with a function object F and Realm Record realm as its argument. It performs the following steps:
"caller"
,
PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false,
[[Configurable]]: true})."arguments"
,
PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false,
[[Configurable]]: true}).The %ThrowTypeError% intrinsic is an anonymous built-in function object that is defined once for each Realm. When %ThrowTypeError% is called it performs the following steps:
The value of the [[Extensible]] internal slot of a %ThrowTypeError% function is false.
The length
property of a %ThrowTypeError% function has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: false }.
The abstract operation MakeConstructor requires a Function argument F and optionally, a
Boolean writablePrototype and an object prototype. If prototype is provided it is assumed
to already contain, if needed, a "constructor"
property whose value is F. This operation converts
F into a constructor by performing the following steps:
prototype
own property."constructor"
, PropertyDescriptor{[[Value]]: F, [[Writable]]: writablePrototype,
[[Enumerable]]: false, [[Configurable]]: true })."prototype"
, PropertyDescriptor{[[Value]]: prototype, [[Writable]]: writablePrototype,
[[Enumerable]]: false, [[Configurable]]: false}).The abstract operation MakeClassConstructor with argument F performs the following steps:
"normal"
."classConstructor"
.The abstract operation MakeMethod with arguments F and homeObject configures F as a method by performing the following steps:
The abstract operation SetFunctionName requires a Function argument F, a String or Symbol
argument name and optionally a String argument prefix. This operation adds a name
property to F by performing the following steps:
name
own property."["
, description, and "]"
."name"
,
PropertyDescriptor{[[Value]]: name, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true}).NOTE 1 When an execution context is established for evaluating an ECMAScript function a new function Environment Record is created and bindings for each formal parameter are instantiated in that Environment Record. Each declaration in the function body is also instantiated. If the function’s formal parameters do not include any default value initializers then the body declarations are instantiated in the same Environment Record as the parameters. If default value parameter initializers exist, a second Environment Record is created for the body declarations. Formal parameters and functions are initialized as part of FunctionDeclarationInstantiation. All other bindings are initialized during evaluation of the function body.
FunctionDeclarationInstantiation is performed as follows using arguments func and argumentsList. func is the function object for which the execution context is being established.
"arguments"
is an element of parameterNames, then
"arguments"
is an element of functionNames or if "arguments"
is an element of
lexicalNames, then
"arguments"
)."arguments"
)."arguments"
, ao)."arguments"
to parameterNames.eval
(see <IP_ADDRESS>) can determine whether any var scoped
declarations introduced by the eval code conflict with pre-existing top-level lexically scoped declarations. This
is not needed for strict functions because a strict direct eval
always places all declarations into a
new Environment Record.NOTE 2 B.3.3 provides an extension to the above algorithm that is necessary for backwards compatibility with web browser implementations of ECMAScript that predate ECMAScript 2015.
The built-in function objects defined in this specification may be implemented as either ECMAScript function objects (9.2) whose behaviour is provided using ECMAScript code or as implementation provided exotic function objects whose behaviour is provided in some other manner. In either case, the effect of calling such functions must conform to their specifications. An implementation may also provide additional built-in function objects that are not defined in this specification.
If a built-in function object is implemented as an exotic object it must have the ordinary object behaviour specified in 9.1. All such exotic function objects also have [[Prototype]], [[Extensible]], and [[Realm]] internal slots.
Unless otherwise specified every built-in function object has the %FunctionPrototype% object (19.2.3) as the initial value of its [[Prototype]] internal slot.
The behaviour specified for each built-in function via algorithm steps or other means is the specification of the
function body behaviour for both [[Call]] and [[Construct]] invocations of the function. However, [[Construct]] invocation
is not supported by all built-in functions. For each built-in function, when invoked with [[Call]], the [[Call]]
thisArgument provides the this value, the [[Call]] argumentsList provides
the named parameters, and the NewTarget value is undefined. When invoked with [[Construct]], the
this value is uninitialized, the [[Construct]] argumentsList provides the named
parameters, and the [[Construct]] newTarget parameter provides the NewTarget value. If the built-in function is
implemented as an ECMAScript function object then this specified behaviour
must be implemented by the ECMAScript code that is the body of the function. Built-in functions that are ECMAScript function
objects must be strict mode functions. If a built-in constructor has any [[Call]] behaviour other than throwing a TypeError exception, an ECMAScript implementation of the function must be done in a manner that does
not cause the function’s [[FunctionKind]] internal slot
to have the value "classConstructor"
.
Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method
unless otherwise specified in the description of a particular function. When a built-in constructor is called as part of a
new
expression the argumentsList parameter of the invoked [[Construct]] internal method provides the
values for the built-in constructor’s named parameters.
Built-in functions that are not constructors do not have a prototype
property unless otherwise specified in
the description of a particular function.
If a built-in function object is not implemented as an ECMAScript function it must provide [[Call]] and [[Construct]] internal methods that conform to the following definitions:
The [[Call]] internal method for a built-in function object F is called with parameters thisArgument and argumentsList, a List of ECMAScript language values. The following steps are taken:
NOTE When calleeContext is removed from the execution context stack it must not be destroyed if it has been suspended and retained by an accessible generator object for later resumption.
The [[Construct]] internal method for built-in function object F is called with parameters argumentsList and newTarget. The steps performed are the same as [[Call]] (see 9.3.1) except that step 9 is replaced by:
9. Let result be the Completion Record that is the result of evaluating F in an implementation defined manner that conforms to the specification of F. The this value is uninitialized, argumentsList provides the named parameters, and newTarget provides the NewTarget value.
The abstract operation CreateBuiltinFunction takes arguments realm, prototype, and steps. The optional argument internalSlotsList is a List of the names of additional internal slots that must be defined as part of the object. If the list is not provided, an empty List is used. CreateBuiltinFunction returns a built-in function object created by the following steps:
Each built-in function defined in this specification is created as if by calling the CreateBuiltinFunction abstract operation, unless otherwise specified.
This specification defines several kinds of built-in exotic objects. These objects generally behave similar to ordinary objects except for a few specific situations. The following exotic objects use the ordinary object internal methods except where it is explicitly specified otherwise below:
A bound function is an exotic object that wraps another function object. A bound function is callable (it has a [[Call]] internal method and may have a [[Construct]] internal method). Calling a bound function generally results in a call of its wrapped function.
Bound function objects do not have the internal slots of ECMAScript function objects defined in Table 27. Instead they have the internal slots defined in Table 28.
| Internal Slot | Type | Description |
|---|---|---|
| [[BoundTargetFunction]] | Callable Object | The wrapped function object. |
| [[BoundThis]] | Any | The value that is always passed as the this value when calling the wrapped function. |
| [[BoundArguments]] | List of Any | A list of values whose elements are used as the first arguments to any call to the wrapped function. |
Unlike ECMAScript function objects, bound function objects do not use an alternative definition of the [[GetOwnProperty]] internal methods. Bound function objects provide all of the essential internal methods as specified in 9.1. However, they use the following definitions for the essential internal methods of function objects.
When the [[Call]] internal method of an exotic bound function object, F, which was created using the bind function is called with parameters thisArgument and argumentsList, a List of ECMAScript language values, the following steps are taken:
When the [[Construct]] internal method of an exotic bound function object, F that was created using the bind function is called with a list of arguments argumentsList and newTarget, the following steps are taken:
The abstract operation BoundFunctionCreate with arguments targetFunction, boundThis and boundArgs is used to specify the creation of new Bound Function exotic objects. It performs the following steps:
An Array object is an exotic object that gives special treatment to array index property keys (see 6.1.7). A property whose property name is an array index is also called an element.
Every Array object has a length
property whose value is always a nonnegative integer less than 232. The value of the length
property is numerically
greater than the name of every own property whose name is an array index; whenever an own property of an Array object is
created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever an own
property is added whose name is an array index, the value of the length
property is changed, if necessary, to
be one more than the numeric value of that array index; and whenever the value of the length
property is
changed, every own property whose name is an array index whose value is not smaller than the new length is deleted. This
constraint applies only to own properties of an Array object and is unaffected by length
or array index
properties that may be inherited from its prototypes.
NOTE A String property name P is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 232−1.
Array exotic objects always have a non-configurable property named "length"
.
Array exotic objects provide an alternative definition for the [[DefineOwnProperty]] internal method. Except for that internal method, Array exotic objects provide all of the other essential internal methods as specified in 9.1.
When the [[DefineOwnProperty]] internal method of an Array exotic object A is called with property key P, and Property Descriptor Desc the following steps are taken:
"length"
, then
"length"
)."length"
, oldLenDesc).The abstract operation ArrayCreate with argument length (a positive integer) and optional argument proto is used to specify the creation of new Array exotic objects. It performs the following steps:
"length"
,
PropertyDescriptor{[[Value]]: length, [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: false}).The abstract operation ArraySpeciesCreate with arguments originalArray and length is used to specify the creation of a new Array object using a constructor function that is derived from originalArray. It performs the following steps:
"constructor"
).NOTE If originalArray was created using the standard built-in Array constructor for a Realm that is not the Realm of the running execution context, then a new Array is created using the Realm of the running execution context. This maintains compatibility with Web browsers that have historically had that behaviour for the Array.prototype methods that now are defined using ArraySpeciesCreate.
When the abstract operation ArraySetLength is called with an Array exotic object A, and Property Descriptor Desc the following steps are taken:
"length"
,
Desc)."length"
)."length"
,
newLenDesc)."length"
, newLenDesc)."length"
, newLenDesc)."length"
,
PropertyDescriptor{[[Writable]]: false}). This call will always return true.NOTE In steps 3 and 4, if Desc.[[Value]] is an object then its valueOf
method is called twice. This is legacy
behaviour that was specified with this effect starting with the 2nd Edition of this specification.
A String object is an exotic object that encapsulates a String value and exposes virtual integer indexed data
properties corresponding to the individual code unit elements of the String value. Exotic String objects always have a
data property named "length"
whose value is the number of code unit elements in the encapsulated String
value. Both the code unit data properties and the "length"
property are non-writable and
non-configurable.
Exotic String objects have the same internal slots as ordinary objects. They also have a [[StringData]] internal slot.
Exotic String objects provide alternative definitions for the following internal methods. All of the other exotic String object essential internal methods that are not defined below are as specified in 9.1.
When the [[GetOwnProperty]] internal method of an exotic String object S is called with property key P the following steps are taken:
When the abstract operation StringGetIndexProperty is called with an exotic String object S and with property key P, the following steps are taken:
When the [[HasProperty]] internal method of an exotic String object S is called with property key P, the following steps are taken:
When the [[OwnPropertyKeys]] internal method of a String exotic object O is called the following steps are taken:
The abstract operation StringCreate with arguments value and prototype is used to specify the creation of new exotic String objects. It performs the following steps:
"length"
,
PropertyDescriptor{[[Value]]: length, [[Writable]]: false, [[Enumerable]]: false,
[[Configurable]]: false }).Most ECMAScript functions make an arguments objects available to their code. Depending upon the characteristics of the function definition, its argument object is either an ordinary object or an arguments exotic object. An arguments exotic object is an exotic object whose array index properties map to the formal parameters bindings of an invocation of its associated ECMAScript function.
Arguments exotic objects have the same internal slots as ordinary objects. They also have a [[ParameterMap]] internal
slot. Ordinary arguments objects also have a [[ParameterMap]] internal slot whose value is always undefined. For ordinary
argument objects the [[ParameterMap]] internal slot is only used by Object.prototype.toString
(<IP_ADDRESS>) to identify them as such.
Arguments exotic objects provide alternative definitions for the following internal methods. All of the other exotic arguments object essential internal methods that are not defined below are as specified in 9.1
NOTE 1 For non-strict functions the integer indexed data properties of an arguments object whose numeric name values are less than the number of formal parameters of the corresponding function object initially share their values with the corresponding argument bindings in the function’s execution context. This means that changing the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed into an accessor property. For strict mode functions, the values of the arguments object’s properties are simply a copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal parameter values.
NOTE 2 The ParameterMap object and its property values are used as a device for specifying the arguments object correspondence to argument bindings. The ParameterMap object and the objects that are the values of its properties are not directly observable from ECMAScript code. An ECMAScript implementation does not need to actually create or use such objects to implement the specified semantics.
NOTE 3 Arguments objects for strict mode functions define non-configurable accessor
properties named "caller"
and "callee"
which throw a TypeError exception on access. The
"callee"
property has a more specific meaning for non-strict functions and a "caller"
property
has historically been provided as an implementation-defined extension by some ECMAScript implementations. The strict
mode definition of these properties exists to ensure that neither of them is defined in any other manner by conforming
ECMAScript implementations.
The [[GetOwnProperty]] internal method of an arguments exotic object when called with a property key P performs the following steps:
"caller"
and desc.[[Value]] is a strict mode Function object, throw a TypeError
exception.If an implementation does not provide a built-in caller
property for argument exotic objects then step 8
of this algorithm is must be skipped.
The [[DefineOwnProperty]] internal method of an arguments exotic object when called with a property key P and Property Descriptor Desc performs the following steps:
The [[Get]] internal method of an arguments exotic object when called with a property key P and ECMAScript language value Receiver performs the following steps:
The [[Set]] internal method of an arguments exotic object when called with property key P, value V, and ECMAScript language value Receiver performs the following steps:
The [[Delete]] internal method of an arguments exotic object when called with a property key P performs the following steps:
The abstract operation CreateUnmappedArgumentsObject called with an argument argumentsList performs the following steps:
"length"
,
PropertyDescriptor{[[Value]]: len, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]:
true})."caller"
,
PropertyDescriptor {[[Get]]: %ThrowTypeError%, [[Set]]: %ThrowTypeError%, [[Enumerable]]: false, [[Configurable]]:
false})."callee"
,
PropertyDescriptor {[[Get]]: %ThrowTypeError%, [[Set]]: %ThrowTypeError%, [[Enumerable]]: false, [[Configurable]]:
false}).The abstract operation CreateMappedArgumentsObject is called with object func, parsed grammar phrase formals, List argumentsList, and Environment Record env. The following steps are performed:
"length"
,
PropertyDescriptor{[[Value]]: len, [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: true})."callee"
,
PropertyDescriptor {[[Value]]: func, [[Writable]]: true, [[Enumerable]]: false,
[[Configurable]]: true}).The abstract operation MakeArgGetter called with String name and Environment Record env creates a built-in function object that when executed returns the value bound for name in env. It performs the following steps:
An ArgGetter function is an anonymous built-in function with [[name]] and [[env]] internal slots. When an ArgGetter function f that expects no arguments is called it performs the following steps:
NOTE ArgGetter functions are never directly accessible to ECMAScript code.
The abstract operation MakeArgSetter called with String name and Environment Record env creates a built-in function object that when executed sets the value bound for name in env. It performs the following steps:
An ArgSetter function is an anonymous built-in function with [[name]] and [[env]] internal slots. When an ArgSetter function f is called with argument value it performs the following steps:
NOTE ArgSetter functions are never directly accessible to ECMAScript code.
An Integer Indexed object is an exotic object that performs special handling of integer index property keys.
Integer Indexed exotic objects have the same internal slots as ordinary objects additionally [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.
Integer Indexed Exotic objects provide alternative definitions for the following internal methods. All of the other Integer Indexed exotic object essential internal methods that are not defined below are as specified in 9.1.
When the [[GetOwnProperty]] internal method of an Integer Indexed exotic object O is called with property key P the following steps are taken:
When the [[HasProperty]] internal method of an Integer Indexed exotic object O is called with property key P, the following steps are taken:
When the [[DefineOwnProperty]] internal method of an Integer Indexed exotic object O is called with property key P, and Property Descriptor Desc the following steps are taken:
When the [[Get]] internal method of an Integer Indexed exotic object O is called with property key P and ECMAScript language value Receiver the following steps are taken:
When the [[Set]] internal method of an Integer Indexed exotic object O is called with property key P, value V, and ECMAScript language value Receiver, the following steps are taken:
When the [[OwnPropertyKeys]] internal method of an Integer Indexed exotic object O is called the following steps are taken:
The abstract operation IntegerIndexedObjectCreate with arguments prototype and internalSlotsList is used to specify the creation of new Integer Indexed exotic objects. The argument internalSlotsList is a List of the names of additional internal slots that must be defined as part of the object. IntegerIndexedObjectCreate performs the following steps:
The abstract operation IntegerIndexedElementGet with arguments O and index performs the following steps:
The abstract operation IntegerIndexedElementSet with arguments O, index, and value performs the following steps:
A module namespace object is an exotic object that exposes the bindings exported from an ECMAScript Module (See 15.2.3). There is a one-to-one correspondence between
the String-keyed own properties of a module namespace exotic object and the binding names exported by the Module. The exported bindings include any bindings that are indirectly exported using export
*
export items. Each String-valued own property key is the StringValue of the
corresponding exported binding name. These are the only String-keyed properties of a module namespace exotic object. Each
such property has the attributes {[[Configurable]]: false, [[Enumerable]]: true}. Module namespace objects are not extensible.
Module namespace objects have the internal slots defined in Table 29.
| Internal Slot | Type | Description |
|---|---|---|
| [[Module]] | Module Record | The Module Record whose exports this namespace exposes. |
| [[Exports]] | List of String | A List containing the String values of the exported names exposed as own properties of this object. The list is ordered as if an Array of those String values had been sorted using Array.prototype.sort using SortCompare as comparefn. |
Module namespace exotic objects provide alternative definitions for all of the internal methods.
When the [[GetPrototypeOf]] internal method of a module namespace exotic object O is called the following steps are taken:
When the [[SetPrototypeOf]] internal method of a module namespace exotic object O is called with argument V the following steps are taken:
When the [[IsExtensible]] internal method of a module namespace exotic object O is called the following steps are taken:
When the [[PreventExtensions]] internal method of a module namespace exotic object O is called the following steps are taken:
When the [[GetOwnProperty]] internal method of a module namespace exotic object O is called with property key P, the following steps are taken:
When the [[DefineOwnProperty]] internal method of a module namespace exotic object O is called with property key P and Property Descriptor Desc, the following steps are taken:
When the [[HasProperty]] internal method of a module namespace exotic object O is called with property key P, the following steps are taken:
When the [[Get]] internal method of a module namespace exotic object O is called with property key P and ECMAScript language value Receiver the following steps are taken:
"ambiguous"
.NOTE ResolveExport is idempotent and side-effect free. An implementation might choose to pre-compute or cache the ResolveExport results for the [[Exports]] of each module namespace exotic object.
When the [[Set]] internal method of a module namespace exotic object O is called with property key P, value V, and ECMAScript language value Receiver, the following steps are taken:
When the [[Delete]] internal method of a module namespace exotic object O is called with property key P the following steps are taken:
When the [[Enumerate]] internal method of a module namespace exotic object O is called the following steps are taken:
When the [[OwnPropertyKeys]] internal method of a module namespace exotic object O is called the following steps are taken:
The abstract operation ModuleNamespaceCreate with arguments module, and exports is used to specify the creation of new module namespace exotic objects. It performs the following steps:
A proxy object is an exotic object whose essential internal methods are partially implemented using ECMAScript code. Every proxy objects has an internal slot called [[ProxyHandler]]. The value of [[ProxyHandler]] is an object, called the proxy’s handler object, or null. Methods (see Table 30) of a handler object may be used to augment the implementation for one or more of the proxy object’s internal methods. Every proxy object also has an internal slot called [[ProxyTarget]] whose value is either an object or the null value. This object is called the proxy’s target object.
| Internal Method | Handler Method |
|---|---|
| [[GetPrototypeOf]] | getPrototypeOf |
| [[SetPrototypeOf]] | setPrototypeOf |
| [[IsExtensible]] | isExtensible |
| [[PreventExtensions]] | preventExtensions |
| [[GetOwnProperty]] | getOwnPropertyDescriptor |
| [[HasProperty]] | has |
| [[Get]] | get |
| [[Set]] | set |
| [[Delete]] | deleteProperty |
| [[DefineOwnProperty]] | defineProperty |
| [[Enumerate]] | enumerate |
| [[OwnPropertyKeys]] | ownKeys |
| [[Call]] | apply |
| [[Construct]] | construct |
When a handler method is called to provide the implementation of a proxy object internal method, the handler method is passed the proxy’s target object as a parameter. A proxy’s handler object does not necessarily have a method corresponding to every essential internal method. Invoking an internal method on the proxy results in the invocation of the corresponding internal method on the proxy’s target object if the handler object does not have a method corresponding to the internal trap.
The [[ProxyHandler]] and [[ProxyTarget]] internal slots of a proxy object are always initialized when the object is created and typically may not be modified. Some proxy objects are created in a manner that permits them to be subsequently revoked. When a proxy is revoked, its [[ProxyHander]] and [[ProxyTarget]] internal slots are set to null causing subsequent invocations of internal methods on that proxy object to throw a TypeError exception.
Because proxy objects permit the implementation of internal methods to be provided by arbitrary ECMAScript code, it is possible to define a proxy object whose handler methods violates the invariants defined in <IP_ADDRESS>. Some of the internal method invariants defined in <IP_ADDRESS> are essential integrity invariants. These invariants are explicitly enforced by the proxy object internal methods specified in this section. An ECMAScript implementation must be robust in the presence of all possible invariant violations.
In the following algorithm descriptions, assume O is an ECMAScript proxy object, P is a property key value, V is any ECMAScript language value and Desc is a Property Descriptor record.
When the [[GetPrototypeOf]] internal method of a Proxy exotic object O is called the following steps are taken:
"getPrototypeOf"
).NOTE [[GetPrototypeOf]] for proxy objects enforces the following invariant:
The result of [[GetPrototypeOf]] must be either an Object or null.
If the target object is not extensible, [[GetPrototypeOf]] applied to the proxy object must return the same value as [[GetPrototypeOf]] applied to the proxy object’s target object.
When the [[SetPrototypeOf]] internal method of a Proxy exotic object O is called with argument V the following steps are taken:
"setPrototypeOf"
).NOTE [[SetPrototypeOf]] for proxy objects enforces the following invariant:
The result of [[SetPrototypeOf]] is a Boolean value.
If the target object is not extensible, the argument value must be the same as the result of [[GetPrototypeOf]] applied to target object.
When the [[IsExtensible]] internal method of a Proxy exotic object O is called the following steps are taken:
"isExtensible"
).NOTE [[IsExtensible]] for proxy objects enforces the following invariant:
The result of [[IsExtensible]] is a Boolean value.
[[IsExtensible]] applied to the proxy object must return the same value as [[IsExtensible]] applied to the proxy object’s target object with the same argument.
When the [[PreventExtensions]] internal method of a Proxy exotic object O is called the following steps are taken:
"preventExtensions"
).NOTE [[PreventExtensions]] for proxy objects enforces the following invariant:
The result of [[PreventExtensions]] is a Boolean value.
[[PreventExtensions]] applied to the proxy object only returns true if [[IsExtensible]] applied to the proxy object’s target object is false.
When the [[GetOwnProperty]] internal method of a Proxy exotic object O is called with property key P, the following steps are taken:
"getOwnPropertyDescriptor"
).NOTE [[GetOwnProperty]] for proxy objects enforces the following invariants:
The result of [[GetOwnProperty]] must be either an Object or undefined.
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.
A property cannot be reported as existent, if it does not exists as an own property of the target object and the target object is not extensible.
A property cannot be reported as non-configurable, if it does not exists as an own property of the target object or if it exists as a configurable own property of the target object.
When the [[DefineOwnProperty]] internal method of a Proxy exotic object O is called with property key P and Property Descriptor Desc, the following steps are taken:
"defineProperty"
).NOTE [[DefineOwnProperty]] for proxy objects enforces the following invariants:
The result of [[DefineOwnProperty]] is a Boolean value.
A property cannot be added, if the target object is not extensible.
A property cannot be non-configurable, unless there exists a corresponding non-configurable own property of the target object.
If a property has a corresponding target object property then applying the Property Descriptor of the property to the target object using [[DefineOwnProperty]] will not throw an exception.
When the [[HasProperty]] internal method of a Proxy exotic object O is called with property key P, the following steps are taken:
"has"
).NOTE [[HasProperty]] for proxy objects enforces the following invariants:
The result of [[HasProperty]] is a Boolean value.
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.
When the [[Get]] internal method of a Proxy exotic object O is called with property key P and ECMAScript language value Receiver the following steps are taken:
"get"
).NOTE [[Get]] for proxy objects enforces the following invariants:
The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable own data property.
The value reported for a property must be undefined if the corresponding target object property is a non-configurable own accessor property that has undefined as its [[Get]] attribute.
When the [[Set]] internal method of a Proxy exotic object O is called with property key P, value V, and ECMAScript language value Receiver, the following steps are taken:
"set"
).NOTE [[Set]] for proxy objects enforces the following invariants:
The result of [[Set]] is a Boolean value.
Cannot change the value of a property to be different from the value of the corresponding target object property if the corresponding target object property is a non-writable, non-configurable own data property.
Cannot set the value of a property if the corresponding target object property is a non-configurable own accessor property that has undefined as its [[Set]] attribute.
When the [[Delete]] internal method of a Proxy exotic object O is called with property key P the following steps are taken:
"deleteProperty"
).NOTE [[Delete]] for proxy objects enforces the following invariant:
When the [[Enumerate]] internal method of a Proxy exotic object O is called the following steps are taken:
"enumerate"
).NOTE [[Enumerate]] for proxy objects enforces the following invariants:
When the [[OwnPropertyKeys]] internal method of a Proxy exotic object O is called the following steps are taken:
"ownKeys"
).NOTE [[OwnPropertyKeys]] for proxy objects enforces the following invariants:
The result of [[OwnPropertyKeys]] is a List.
The Type of each result List element is either String or Symbol.
The result List must contain the keys of all non-configurable own properties of the target object.
If the target object is not extensible, then the result List must contain all the keys of the own properties of the target object and no other values.
The [[Call]] internal method of a Proxy exotic object O is called with parameters thisArgument and argumentsList, a List of ECMAScript language values. The following steps are taken:
"apply"
).NOTE A Proxy exotic object only has a [[Call]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Call]] internal method.
The [[Construct]] internal method of a Proxy exotic object O is called with parameters argumentsList which is a possibly empty List of ECMAScript language values and newTarget. The following steps are taken:
"construct"
).NOTE 1 A Proxy exotic object only has a [[Construct]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Construct]] internal method.
NOTE 2 [[Construct]] for proxy objects enforces the following invariants:
The abstract operation ProxyCreate with arguments target and handler is used to specify the creation of new Proxy exotic objects. It performs the following steps:
ECMAScript code is expressed using Unicode, version 5.1 or later. ECMAScript source text is a sequence of code points. All Unicode code point values from U+0000 to U+10FFFF, including surrogate code points, may occur in source text where permitted by the ECMAScript grammars. The actual encodings used to store and interchange ECMAScript source text is not relevant to this specification. Regardless of the external source text encoding, a conforming ECMAScript implementation processes the source text as if it was an equivalent sequence of SourceCharacter values. Each SourceCharacter being a Unicode code point. Conforming ECMAScript implementations are not required to perform any normalization of source text, or behave as though they were performing normalization of source text.
The components of a combining character sequence are treated as individual Unicode code points even though a user might think of the whole sequence as a single character.
NOTE In string literals, regular expression literals, template literals and identifiers, any Unicode code point may also be expressed using Unicode escape sequences that explicitly express a code point’s numeric value. Within a comment, such an escape sequence is effectively ignored as part of the comment.
ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a Java program,
if the Unicode escape sequence \u000A
, for example, occurs within a single-line comment, it is interpreted as
a line terminator (Unicode code point U+000A is LINE FEED (LF) and therefore the next code point is not part of the
comment. Similarly, if the Unicode escape sequence \u000A
occurs within a string literal in a Java program,
it is likewise interpreted as a line terminator, which is not allowed within a string literal—one must write
\n
instead of \u000A
to cause a LINE FEED (LF) to be part of the String value of a string
literal. In an ECMAScript program, a Unicode escape sequence occurring within a comment is never interpreted and therefore
cannot contribute to termination of the comment. Similarly, a Unicode escape sequence occurring within a string literal in
an ECMAScript program always contributes to the literal and is never interpreted as a line terminator or as a code point
that might terminate the string literal.
The UTF16Encoding of a numeric code point value, cp, is determined as follows:
Two code units, lead and trail, that form a UTF-16 surrogate pair are converted to a code point by performing the following steps:
There are four types of ECMAScript code:
Global code is source text that is treated as an ECMAScript Script. The global code of a particular Script does not include any source text that is parsed as part of a FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, MethodDefinition, ArrowFunction, ClassDeclaration, or ClassExpression.
Eval code is the source text supplied to the built-in eval
function. More precisely, if the
parameter to the built-in eval
function is a String, it is treated as an ECMAScript Script. The eval
code for a particular invocation of eval
is the global code portion of that Script.
Function code is source text that is parsed to supply the value of the [[ECMAScriptCode]] and [[FormalParameters]] internal slots (see 9.2) of an ECMAScript function object. The function code of a particular ECMAScript function does not include any source text that is parsed as the function code of a nested FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, MethodDefinition, ArrowFunction, ClassDeclaration, or ClassExpression.
Module code is source text that is code that is provided as a ModuleBody. It is the code that is directly evaluated when a module is initialized. The module code of a particular module does not include any source text that is parsed as part of a nested FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, MethodDefinition, ArrowFunction, ClassDeclaration, or ClassExpression.
NOTE Function code is generally provided as the bodies of Function Definitions (14.1), Arrow Function Definitions (14.2), Method Definitions (14.3) and
Generator Definitions (14.4). Function code is also derived from the
arguments to the Function
constructor (<IP_ADDRESS>) and the
GeneratorFunction constructor (<IP_ADDRESS>).
An ECMAScript Script syntactic unit may be processed using either unrestricted or strict mode syntax and semantics. Code is interpreted as strict mode code in the following situations:
Global code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive (see 14.1.1).
Module code is always strict mode code.
All parts of a ClassDeclaration or a ClassExpression are strict mode code.
Eval code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive or if the call to eval is a direct eval (see <IP_ADDRESS>) that is contained in strict mode code.
Function code is strict mode code if the associated FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, MethodDefinition, or ArrowFunction is contained in strict mode code or if the code that produces the value of the function’s [[ECMAScriptCode]] internal slot begins with a Directive Prologue that contains a Use Strict Directive.
Function code that is supplied as the arguments to the built-in Function
and Generator
constructors is strict mode code if the last argument is a String that when processed is a FunctionBody that begins with a Directive Prologue that contains a Use Strict Directive.
ECMAScript code that is not strict mode code is called non-strict code.
An ECMAScript implementation may support the evaluation of exotic function objects whose evaluative behaviour is expressed in some implementation defined form of executable code other than via ECMAScript code. Whether a function object is an ECMAScript code function or a non-ECMAScript function is not semantically observable from the perspective of an ECMAScript code function that calls or is called by such a non-ECMAScript function.
The source text of an ECMAScript Script or Module is first converted into a sequence of input elements, which are tokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible sequence of code points as the next input element.
There are several situations where the identification of lexical input elements is sensitive to the syntactic grammar context that is consuming the input elements. This requires multiple goal symbols for the lexical grammar. The InputElementRegExpOrTemplateTail goal is used in syntactic grammar contexts where a RegularExpressionLiteral, a TemplateMiddle, or a TemplateTail is permitted. The InputElementRegExp goal symbol is used in all syntactic grammar contexts where a RegularExpressionLiteral is permitted but neither a TemplateMiddle, nor a TemplateTail is permitted. The InputElementTemplateTail goal is used in all syntactic grammar contexts where a TemplateMiddle or a TemplateTail is permitted but a RegularExpressionLiteral is not permitted. In all other contexts, InputElementDiv is used as the lexical goal symbol.
NOTE The use of multiple lexical goals ensures that there are no lexical ambiguities that would affect automatic semicolon insertion. For example, there are no syntactic grammar contexts where both a leading division or division-assignment, and a leading RegularExpressionLiteral are permitted. This is not affected by semicolon insertion (see 11.9); in examples such as the following:
a = b
/hi/g.exec(c).map(d);
where the first non-whitespace, non-comment code point after a LineTerminator is U+002F (SOLIDUS) and the syntactic context allows division or division-assignment, no semicolon is inserted at the LineTerminator. That is, the above example is interpreted in the same way as:
a = b / hi / g.exec(c).map(d);
The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character Database such as LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK) are control codes used to control the formatting of a range of text in the absence of higher-level protocols for this (such as mark-up languages).
It is useful to allow format-control characters in source text to facilitate editing and display. All format control characters may be used within comments, and within string literals, template literals, and regular expression literals.
U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are format-control characters that are used to make necessary distinctions when forming words or phrases in certain languages. In ECMAScript source text these code points may also be used in an IdentifierName (see 11.6.1) after the first character.
U+FEFF (ZERO WIDTH NO-BREAK SPACE) is a format-control character used primarily at the start of a text to mark it as Unicode and to allow detection of the text's encoding and byte order. <ZWNBSP> characters intended for this purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. In ECMAScript source text <ZWNBSP> code points are treated as white space characters (see 11.2).
The special treatment of certain format-control characters outside of comments, string literals, and regular expression literals is summarized in Table 31.
| Code Point | Name | Abbreviation | Usage |
|---|---|---|---|
U+200C |
ZERO WIDTH NON-JOINER | <ZWNJ> | IdentifierPart |
U+200D |
ZERO WIDTH JOINER | <ZWJ> | IdentifierPart |
U+FEFF |
ZERO WIDTH NO-BREAK SPACE | <ZWNBSP> | WhiteSpace |
White space code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space code points may occur between any two tokens and at the start or end of input. White space code points may occur within a StringLiteral, a RegularExpressionLiteral, a Template, or a TemplateSubstitutionTail where they are considered significant code points forming part of a literal value. They may also occur within a Comment, but cannot appear within any other kind of token.
The ECMAScript white space code points are listed in Table 32.
| Code Point | Name | Abbreviation |
|---|---|---|
U+0009 |
CHARACTER TABULATION | <TAB> |
U+000B |
LINE TABULATION | <VT> |
U+000C |
FORM FEED (FF) | <FF> |
U+0020 |
SPACE | <SP> |
U+00A0 |
NO-BREAK SPACE | <NBSP> |
U+FEFF |
ZERO WIDTH NO-BREAK SPACE | <ZWNBSP> |
| Other category “Zs” | Any other Unicode “Separator, space” code point | <USP> |
ECMAScript implementations must recognize as WhiteSpace code points listed in the “Separator, space” (Zs) category by Unicode 5.1. ECMAScript implementations may also recognize as WhiteSpace additional category Zs code points from subsequent editions of the Unicode Standard.
NOTE Other than for the code points listed in Table 32, ECMAScript WhiteSpace intentionally excludes all code points that have the Unicode “White_Space” property but which are not classified in category “Zs”.
Like white space code points, line terminator code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space code points, line terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators may occur between any two tokens, but there are a few places where they are forbidden by the syntactic grammar. Line terminators also affect the process of automatic semicolon insertion (11.9). A line terminator cannot occur within any token except a StringLiteral, Template, or TemplateSubstitutionTail. Line terminators may only occur within a StringLiteral token as part of a LineContinuation.
A line terminator can occur within a MultiLineComment (11.4) but cannot occur within a SingleLineComment.
Line terminators are included in the set of white space code points that are matched by the \s
class in
regular expressions.
The ECMAScript line terminator code points are listed in Table 33.
| Code Point | Unicode Name | Abbreviation |
|---|---|---|
U+000A |
LINE FEED (LF) | <LF> |
U+000D |
CARRIAGE RETURN (CR) | <CR> |
U+2028 |
LINE SEPARATOR | <LS> |
U+2029 |
PARAGRAPH SEPARATOR | <PS> |
Only the Unicode code points in Table 33 are treated as line terminators. Other new line or line breaking Unicode code points are not treated as line terminators but are treated as white space if they meet the requirements listed in Table 32. The sequence <CR><LF> is commonly used as a line terminator. It should be considered a single SourceCharacter for the purpose of reporting line numbers.
Comments can be either single or multi-line. Multi-line comments cannot nest.
Because a single-line comment can contain any Unicode code point except a LineTerminator code
point, and because of the general rule that a token is always as long as possible, a single-line comment always consists of
all code points from the //
marker to the end of the line. However, the LineTerminator at
the end of the line is not considered to be part of the single-line comment; it is recognized separately by the lexical
grammar and becomes part of the stream of input elements for the syntactic grammar. This point is very important, because it
implies that the presence or absence of single-line comments does not affect the process of automatic semicolon insertion (see
11.9).
Comments behave like white space and are discarded except that, if a MultiLineComment contains a line terminator code point, then the entire comment is considered to be a LineTerminator for purposes of parsing by the syntactic grammar.
/*
MultiLineCommentCharsopt */
*
PostAsteriskCommentCharsopt*
PostAsteriskCommentCharsopt*
/
or *
//
SingleLineCommentCharsoptNOTE The DivPunctuator, RegularExpressionLiteral, RightBracePunctuator, and TemplateSubstitutionTail productions derive additional tokens that are not included in the CommonToken production.
IdentifierName and ReservedWord are tokens that are interpreted according to the Default Identifier Syntax given in Unicode Standard Annex #31, Identifier and Pattern Syntax, with some small modifications. ReservedWord is an enumerated subset of IdentifierName. The syntactic grammar defines Identifier as an IdentifierName that is not a ReservedWord (see 11.6.2). The Unicode identifier grammar is based on character properties specified by the Unicode Standard. The Unicode code points in the specified categories in version 5.1.0 of the Unicode standard must be treated as in those categories by all conforming ECMAScript implementations. ECMAScript implementations may recognize identifier code points defined in later editions of the Unicode Standard.
NOTE 1 This standard specifies specific code point additions: U+0024 (DOLLAR SIGN) and U+005F (LOW LINE) are permitted anywhere in an IdentifierName, and the code points U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are permitted anywhere after the first code point of an IdentifierName.
Unicode escape sequences are permitted in an IdentifierName, where they contribute a single
Unicode code point to the IdentifierName. The code point is expressed by the HexDigits of the UnicodeEscapeSequence (see 11.8.4). The \
preceding the UnicodeEscapeSequence and the u
and { }
code units, if they appear, do not
contribute code points to the IdentifierName. A UnicodeEscapeSequence cannot
be used to put a code point into an IdentifierName that would otherwise be illegal. In other words,
if a \
UnicodeEscapeSequence sequence were replaced by the SourceCharacter it contributes, the result must still be a valid IdentifierName
that has the exact same sequence of SourceCharacter elements as the original IdentifierName. All interpretations of IdentifierName within this specification
are based upon their actual code points regardless of whether or not an escape sequence was used to contribute any
particular code point.
Two IdentifierName that are canonically equivalent according to the Unicode standard are not equal unless, after replacement of each UnicodeEscapeSequence, they are represented by the exact same sequence of code points.
$
_
\
UnicodeEscapeSequence$
_
\
UnicodeEscapeSequenceThe definitions of the nonterminal UnicodeEscapeSequence is given in 11.8.4.
NOTE 2 The sets of code points with Unicode properties “ID_Start” and “ID_Continue” include, respectively, the code points with Unicode properties “Other_ID_Start” and “Other_ID_Continue”.
\
UnicodeEscapeSequenceIt is a Syntax Error if SV(UnicodeEscapeSequence) is none
of "$"
, or "_"
, or the UTF16Encoding (10.1.1) of a code point matched by the UnicodeIDStart lexical
grammar production.
\
UnicodeEscapeSequenceIt is a Syntax Error if SV(UnicodeEscapeSequence) is none
of "$"
, or "_"
, or the UTF16Encoding (10.1.1) of either <ZWNJ> or <ZWJ>, or the UTF16Encoding of a Unicode code point that would be matched by the UnicodeIDContinue lexical grammar production.
\
UnicodeEscapeSequence are first replaced with the
code point represented by the UnicodeEscapeSequence and then the code points of the entire
IdentifierName are converted to code units by UTF16Encoding (10.1.1) each code point.A reserved word is an IdentifierName that cannot be used as an Identifier.
NOTE The ReservedWord definitions are specified as literal sequences
of specific SourceCharacter elements. A code point in a ReservedWord
cannot be expressed by a \
UnicodeEscapeSequence.
The following tokens are ECMAScript keywords and may not be used as Identifiers in ECMAScript programs.
break |
do |
in |
typeof |
case |
else |
instanceof |
var |
catch |
export |
new |
void |
class |
extends |
return |
while |
const |
finally |
super |
with |
continue |
for |
switch |
yield |
debugger |
function |
this |
|
default |
if |
throw |
|
delete |
import |
try |
The following tokens are reserved for used as keywords in future language extensions.
enum
await
await
is only treated as a FutureReservedWord when Module
is the goal symbol of the syntactic grammar.
NOTE Use of the following tokens within strict mode code (see 10.2.1) is also reserved. That usage is restricted using static semantic restrictions (see 12.1.1) rather than the lexical grammar:
implements |
package |
protected |
|
interface |
private |
public |
{ |
( |
) |
[ |
] |
. |
... |
; |
, |
< |
> |
<= |
>= |
== |
!= |
=== |
!== |
|
+ |
- |
* |
% |
++ |
-- |
<< |
>> |
>>> |
& |
| |
^ |
! |
~ |
&& |
|| |
? |
: |
= |
+= |
-= |
*= |
%= |
<<= |
>>= |
>>>= |
&= |
|= |
^= |
=> |
/
/=
}
null
true
false
.
DecimalDigitsopt ExponentPartopt.
DecimalDigits ExponentPartopt0
0
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
e
E
+
DecimalDigits-
DecimalDigits0b
BinaryDigits0B
BinaryDigits0
1
0o
OctalDigits0O
OctalDigits0
1
2
3
4
5
6
7
0x
HexDigits0X
HexDigits0
1
2
3
4
5
6
7
8
9
a
b
c
d
e
f
A
B
C
D
E
F
The SourceCharacter immediately following a NumericLiteral must not be an IdentifierStart or DecimalDigit.
NOTE For example:
3in
is an
error and not the two input elements 3
and in
.
A conforming implementation, when processing strict mode code (see 10.2.1), must not extend, as described in B.1.1, the syntax of NumericLiteral to include LegacyOctalIntegerLiteral, nor extend the syntax of DecimalIntegerLiteral to include NonOctalDecimalIntegerLiteral.
A numeric literal stands for a value of the Number type. This value is determined in two steps: first, a mathematical value (MV) is derived from the literal; second, this mathematical value is rounded as described below.
The MV of NumericLiteral :: DecimalLiteral is the MV of DecimalLiteral.
The MV of NumericLiteral :: BinaryIntegerLiteral is the MV of BinaryIntegerLiteral.
The MV of NumericLiteral :: OctalIntegerLiteral is the MV of OctalIntegerLiteral.
The MV of NumericLiteral :: HexIntegerLiteral is the MV of HexIntegerLiteral.
The MV of DecimalLiteral :: DecimalIntegerLiteral .
is the MV of DecimalIntegerLiteral.
The MV of DecimalLiteral :: DecimalIntegerLiteral .
DecimalDigits is the
MV of DecimalIntegerLiteral plus (the MV of DecimalDigits × 10–n), where
n is the number of code points in DecimalDigits.
The MV of DecimalLiteral :: DecimalIntegerLiteral .
ExponentPart is the MV
of DecimalIntegerLiteral × 10e, where e is the MV of ExponentPart.
The MV of DecimalLiteral :: DecimalIntegerLiteral .
DecimalDigits ExponentPart is (the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits
× 10–n)) × 10e, where n is the number of code points in
DecimalDigits and e is the MV of ExponentPart.
The MV of DecimalLiteral :: .
DecimalDigits is the MV of DecimalDigits ×
10–n, where n is the number of code points in DecimalDigits.
The MV of DecimalLiteral :: .
DecimalDigits ExponentPart is the MV of
DecimalDigits × 10e–n, where n is the number of code points in
DecimalDigits and e is the MV of ExponentPart.
The MV of DecimalLiteral :: DecimalIntegerLiteral is the MV of DecimalIntegerLiteral.
The MV of DecimalLiteral :: DecimalIntegerLiteral ExponentPart is the MV of DecimalIntegerLiteral × 10e, where e is the MV of ExponentPart.
The MV of DecimalIntegerLiteral :: 0
is 0.
The MV of DecimalIntegerLiteral :: NonZeroDigit is the MV of NonZeroDigit.
The MV of DecimalIntegerLiteral :: NonZeroDigit DecimalDigits is (the MV of NonZeroDigit × 10n) plus the MV of DecimalDigits, where n is the number of code points in DecimalDigits.
The MV of DecimalDigits :: DecimalDigit is the MV of DecimalDigit.
The MV of DecimalDigits :: DecimalDigits DecimalDigit is (the MV of DecimalDigits × 10) plus the MV of DecimalDigit.
The MV of ExponentPart :: ExponentIndicator SignedInteger is the MV of SignedInteger.
The MV of SignedInteger :: DecimalDigits is the MV of DecimalDigits.
The MV of SignedInteger :: +
DecimalDigits is the MV of DecimalDigits.
The MV of SignedInteger :: -
DecimalDigits is the negative of the MV of DecimalDigits.
The MV of DecimalDigit :: 0
or of HexDigit :: 0
or of OctalDigit ::
0
or of BinaryDigit :: 0
is 0.
The MV of DecimalDigit :: 1
or of NonZeroDigit ::
1
or of HexDigit ::
1
or of OctalDigit :: 1
or
of BinaryDigit
:: 1
is 1.
The MV of DecimalDigit :: 2
or of NonZeroDigit ::
2
or of HexDigit ::
2
or of OctalDigit :: 2
is 2.
The MV of DecimalDigit :: 3
or of NonZeroDigit ::
3
or of HexDigit ::
3
or of OctalDigit :: 3
is 3.
The MV of DecimalDigit :: 4
or of NonZeroDigit ::
4
or of HexDigit ::
4
or of OctalDigit :: 4
is 4.
The MV of DecimalDigit :: 5
or of NonZeroDigit ::
5
or of HexDigit ::
5
or of OctalDigit :: 5
is 5.
The MV of DecimalDigit :: 6
or of NonZeroDigit ::
6
or of HexDigit ::
6
or of OctalDigit :: 6
is 6.
The MV of DecimalDigit :: 7
or of NonZeroDigit ::
7
or of HexDigit ::
7
or of OctalDigit :: 7
is 7.
The MV of DecimalDigit :: 8
or of NonZeroDigit ::
8
or of HexDigit ::
8
is 8.
The MV of DecimalDigit :: 9
or of NonZeroDigit ::
9
or of HexDigit ::
9
is 9.
The MV of HexDigit :: a
or of HexDigit :: A
is 10.
The MV of HexDigit :: b
or of HexDigit :: B
is 11.
The MV of HexDigit :: c
or of HexDigit :: C
is 12.
The MV of HexDigit :: d
or of HexDigit :: D
is 13.
The MV of HexDigit :: e
or of HexDigit :: E
is 14.
The MV of HexDigit :: f
or of HexDigit :: F
is 15.
The MV of BinaryIntegerLiteral :: 0b
BinaryDigits is the MV of BinaryDigits.
The MV of BinaryIntegerLiteral :: 0B
BinaryDigits is the MV of BinaryDigits.
The MV of BinaryDigits :: BinaryDigit is the MV of BinaryDigit.
The MV of BinaryDigits :: BinaryDigits BinaryDigit is (the MV of BinaryDigits × 2) plus the MV of BinaryDigit.
The MV of OctalIntegerLiteral :: 0o
OctalDigits is the MV of OctalDigits.
The MV of OctalIntegerLiteral :: 0O
OctalDigits is the MV of OctalDigits.
The MV of OctalDigits :: OctalDigit is the MV of OctalDigit.
The MV of OctalDigits :: OctalDigits OctalDigit is (the MV of OctalDigits × 8) plus the MV of OctalDigit.
The MV of HexIntegerLiteral :: 0x
HexDigits is the MV of HexDigits.
The MV of HexIntegerLiteral :: 0X
HexDigits is the MV of HexDigits.
The MV of HexDigits :: HexDigit is the MV of HexDigit.
The MV of HexDigits :: HexDigits HexDigit is (the MV of HexDigits × 16) plus the MV of HexDigit.
Once the exact MV for a numeric literal has been determined, it is then rounded to a value of the Number type. If the
MV is 0, then the rounded value is +0; otherwise, the rounded value must be the Number value
for the MV (as specified in 6.1.6), unless the literal is a DecimalLiteral and the literal has more than 20 significant digits, in which case the Number value may
be either the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a
0
digit or the Number value for the MV of a literal produced by replacing each significant digit after the
20th with a 0
digit and then incrementing the literal at the 20th significant digit position. A digit is
significant if it is not part of an ExponentPart and
0
; orNOTE 1 A string literal is zero or more Unicode code points enclosed in single or double quotes. Unicode code points may also be represented by an escape sequence. All code points may appear literally in a string literal except for the closing quote code points, U+005C (REVERSE SOLIDUS), U+000D (CARRIAGE RETURN), U+2028 (LINE SEPARATOR), U+2029 (PARAGRAPH SEPARATOR), and U+000A (LINE FEED). Any code points may appear in the form of an escape sequence. String literals evaluate to ECMAScript String values. When generating these String values Unicode code points are UTF-16 encoded as defined in 10.1.1. Code points belonging to the Basic Multilingual Plane are encoded as a single code unit element of the string. All other code points are encoded as two code unit elements of the string.
"
DoubleStringCharactersopt "
'
SingleStringCharactersopt '
"
or \
or LineTerminator\
EscapeSequence'
or \
or LineTerminator\
EscapeSequence\
LineTerminatorSequence0
[lookahead ∉ DecimalDigit]A conforming implementation, when processing strict mode code (see 10.2.1), must not extend the syntax of EscapeSequence to include LegacyOctalEscapeSequence as described in B.1.2.
'
"
\
b
f
n
r
t
v
x
u
x
HexDigit HexDigitu
Hex4Digitsu{
HexDigits }
The definition of the nonterminal HexDigit is given in 11.8.3. SourceCharacter is defined in 10.1.
NOTE 2 A line terminator code point cannot appear in a string literal, except as part of a
LineContinuation to produce the empty code points sequence. The proper way to cause a line
terminator code point to be part of the String value of a string literal is to use an escape sequence such as
\n
or \u000A
.
u{
HexDigits }
"
DoubleStringCharactersopt "
'
SingleStringCharactersopt '
A string literal stands for a value of the String type. The String value (SV) of the literal is described in terms of code unit values contributed by the various parts of the string literal. As part of this process, some Unicode code points within the string literal are interpreted as having a mathematical value (MV), as described below or in 11.8.3.
The SV of StringLiteral :: ""
is the empty code unit sequence.
The SV of StringLiteral :: ''
is the empty code unit sequence.
The SV of StringLiteral :: "
DoubleStringCharacters "
is the SV of
DoubleStringCharacters.
The SV of StringLiteral :: '
SingleStringCharacters '
is the SV of
SingleStringCharacters.
The SV of DoubleStringCharacters :: DoubleStringCharacter is a sequence of one or two code units that is the SV of DoubleStringCharacter.
The SV of DoubleStringCharacters :: DoubleStringCharacter DoubleStringCharacters is a sequence of one or two code units that is the SV of DoubleStringCharacter followed by all the code units in the SV of DoubleStringCharacters in order.
The SV of SingleStringCharacters :: SingleStringCharacter is a sequence of one or two code units that is the SV of SingleStringCharacter.
The SV of SingleStringCharacters :: SingleStringCharacter SingleStringCharacters is a sequence of one or two code units that is the SV of SingleStringCharacter followed by all the code units in the SV of SingleStringCharacters in order.
The SV of DoubleStringCharacter :: SourceCharacter but not one of "
or \
or LineTerminator is the UTF16Encoding (10.1.1) of the code point value of SourceCharacter.
The SV of DoubleStringCharacter :: \
EscapeSequence is the SV of the EscapeSequence.
The SV of DoubleStringCharacter :: LineContinuation is the empty code unit sequence.
The SV of SingleStringCharacter :: SourceCharacter but not one of '
or \
or LineTerminator is the UTF16Encoding (10.1.1) of the code point value of SourceCharacter.
The SV of SingleStringCharacter :: \
EscapeSequence is the SV of the EscapeSequence.
The SV of SingleStringCharacter :: LineContinuation is the empty code unit sequence.
The SV of EscapeSequence :: CharacterEscapeSequence is the SV of the CharacterEscapeSequence.
The SV of EscapeSequence :: 0
is the code unit value 0.
The SV of EscapeSequence :: HexEscapeSequence is the SV of the HexEscapeSequence.
The SV of EscapeSequence :: UnicodeEscapeSequence is the SV of the UnicodeEscapeSequence.
The SV of CharacterEscapeSequence :: SingleEscapeCharacter is the code unit whose value is determined by the SingleEscapeCharacter according to Table 34.
| Escape Sequence | Code Unit Value | Unicode Character Name | Symbol |
|---|---|---|---|
\b |
0x0008 |
BACKSPACE | <BS> |
\t |
0x0009 |
CHARACTER TABULATION | <HT> |
\n |
0x000A |
LINE FEED (LF) | <LF> |
\v |
0x000B |
LINE TABULATION | <VT> |
\f |
0x000C |
FORM FEED (FF) | <FF> |
\r |
0x000D |
CARRIAGE RETURN (CR) | <CR> |
\" |
0x0022 |
QUOTATION MARK | " |
\' |
0x0027 |
APOSTROPHE | ' |
\\ |
0x005C |
REVERSE SOLIDUS | \ |
The SV of CharacterEscapeSequence :: NonEscapeCharacter is the SV of the NonEscapeCharacter.
The SV of NonEscapeCharacter :: SourceCharacter but not one of EscapeCharacter or LineTerminator is the UTF16Encoding (10.1.1) of the code point value of SourceCharacter.
The SV of HexEscapeSequence :: x
HexDigit HexDigit is the code unit value
that is (16 times the MV of the first HexDigit) plus the MV of the second HexDigit.
The SV of UnicodeEscapeSequence :: u
Hex4Digits is the SV of Hex4Digits.
The SV of Hex4Digits :: HexDigit HexDigit HexDigit HexDigit is the code unit value that is (4096 times the MV of the first HexDigit) plus (256 times the MV of the second HexDigit) plus (16 times the MV of the third HexDigit) plus the MV of the fourth HexDigit.
The SV of UnicodeEscapeSequence :: u{
HexDigits }
is the UTF16Encoding (10.1.1) of the MV of
HexDigits.
NOTE 1 A regular expression literal is an input element that is converted to a RegExp object
(see 21.2) each time the literal is evaluated. Two regular
expression literals in a program evaluate to regular expression objects that never compare as ===
to each
other even if the two literals' contents are identical. A RegExp object may also be created at runtime by new
RegExp
or calling the RegExp
constructor as a function (see
21.2.3).
The productions below describe the syntax for a regular expression literal and are used by the input element scanner to find the end of the regular expression literal. The source text comprising the RegularExpressionBody and the RegularExpressionFlags are subsequently parsed again using the more stringent ECMAScript Regular Expression grammar (21.2.1).
An implementation may extend the ECMAScript Regular Expression grammar defined in 21.2.1, but it must not extend the RegularExpressionBody and RegularExpressionFlags productions defined below or the productions used by these productions.
/
RegularExpressionBody /
RegularExpressionFlags*
or \
or /
or [
\
or /
or [
\
RegularExpressionNonTerminator[
RegularExpressionClassChars ]
]
or \
NOTE 2 Regular expression literals may not be empty; instead of representing an empty regular
expression literal, the code unit sequence //
starts a single-line comment. To specify an empty regular
expression, use: /(?:)/
.
/
RegularExpressionBody /
RegularExpressionFlags/
RegularExpressionBody /
RegularExpressionFlags`
TemplateCharactersopt `
`
TemplateCharactersopt ${
}
TemplateCharactersopt ${
}
TemplateCharactersopt `
$
[lookahead ≠ { ]\
EscapeSequence`
or \
or $
or LineTerminatorA conforming implementation must not use the extended definition of EscapeSequence described in B.1.2 when parsing a TemplateCharacter.
NOTE TemplateSubstitutionTail is used by the InputElementTemplateTail alternative lexical goal.
A template literal component is interpreted as a sequence of Unicode code points. The Template Value (TV) of a literal component is described in terms of code unit values (SV, 11.8.4) contributed by the various parts of the template literal component. As part of this process, some Unicode code points within the template component are interpreted as having a mathematical value (MV, 11.8.3). In determining a TV, escape sequences are replaced by the UTF-16 code unit(s) of the Unicode code point represented by the escape sequence. The Template Raw Value (TRV) is similar to a Template Value with the difference that in TRVs escape sequences are interpreted literally.
The TV and TRV of NoSubstitutionTemplate ::
``
is the empty code unit sequence.
The TV and TRV of TemplateHead :: `${
is the empty code unit sequence.
The TV and TRV of TemplateMiddle :: }${
is the empty code unit sequence.
The TV and TRV of TemplateTail :: }`
is the empty code unit sequence.
The TV of NoSubstitutionTemplate :: `
TemplateCharacters `
is the TV of
TemplateCharacters.
The TV of TemplateHead :: `
TemplateCharacters ${
is the TV of
TemplateCharacters.
The TV of TemplateMiddle :: }
TemplateCharacters ${
is the TV of
TemplateCharacters.
The TV of TemplateTail :: }
TemplateCharacters `
is the TV of
TemplateCharacters.
The TV of TemplateCharacters :: TemplateCharacter is the TV of TemplateCharacter.
The TV of TemplateCharacters :: TemplateCharacter TemplateCharacters is a sequence consisting of the code units in the TV of TemplateCharacter followed by all the code units in the TV of TemplateCharacters in order.
The TV of TemplateCharacter :: SourceCharacter but not one of `
or \
or $
or LineTerminator is the UTF16Encoding (10.1.1) of the code point value of
SourceCharacter.
The TV of TemplateCharacter :: $
is the code unit value 0x0024.
The TV of TemplateCharacter :: \
EscapeSequence is the SV of EscapeSequence.
The TV of TemplateCharacter :: LineContinuation is the TV of LineContinuation.
The TV of TemplateCharacter :: LineTerminatorSequence is the TRV of LineTerminatorSequence.
The TV of LineContinuation :: \
LineTerminatorSequence is the empty code unit sequence.
The TRV of NoSubstitutionTemplate :: `
TemplateCharacters `
is the TRV of
TemplateCharacters.
The TRV of TemplateHead :: `
TemplateCharacters ${
is the TRV of
TemplateCharacters.
The TRV of TemplateMiddle :: }
TemplateCharacters ${
is the TRV of
TemplateCharacters.
The TRV of TemplateTail :: }
TemplateCharacters `
is the TRV of
TemplateCharacters.
The TRV of TemplateCharacters :: TemplateCharacter is the TRV of TemplateCharacter.
The TRV of TemplateCharacters :: TemplateCharacter TemplateCharacters is a sequence consisting of the code units in the TRV of TemplateCharacter followed by all the code units in the TRV of TemplateCharacters, in order.
The TRV of TemplateCharacter :: SourceCharacter but not one of `
or \
or $
or LineTerminator is the UTF16Encoding (10.1.1) of the code point value of
SourceCharacter.
The TRV of TemplateCharacter :: $
is the code unit value 0x0024.
The TRV of TemplateCharacter :: \
EscapeSequence is the sequence consisting of the code unit value
0x005C followed by the code units of TRV of EscapeSequence.
The TRV of TemplateCharacter :: LineContinuation is the TRV of LineContinuation.
The TRV of TemplateCharacter :: LineTerminatorSequence is the TRV of LineTerminatorSequence.
The TRV of EscapeSequence :: CharacterEscapeSequence is the TRV of the CharacterEscapeSequence.
The TRV of EscapeSequence :: 0
is the code unit value 0x0030.
The TRV of EscapeSequence :: HexEscapeSequence is the TRV of the HexEscapeSequence.
The TRV of EscapeSequence :: UnicodeEscapeSequence is the TRV of the UnicodeEscapeSequence.
The TRV of CharacterEscapeSequence :: SingleEscapeCharacter is the TRV of the SingleEscapeCharacter.
The TRV of CharacterEscapeSequence :: NonEscapeCharacter is the SV of the NonEscapeCharacter.
The TRV of SingleEscapeCharacter :: one of '
"
\
b
f
n
r
t
v
is the SV of the SourceCharacter that is that single code point.
The TRV of HexEscapeSequence :: x
HexDigit HexDigit is the sequence
consisting of code unit value 0x0078 followed by TRV of the first HexDigit followed by the TRV of the second
HexDigit.
The TRV of UnicodeEscapeSequence :: u
Hex4Digits is the sequence consisting of code unit value 0x0075
followed by TRV of Hex4Digits.
The TRV of UnicodeEscapeSequence :: u{
HexDigits }
is the sequence consisting of
code unit value 0x0075 followed by code unit value 0x007B followed by TRV of HexDigits followed by code unit
value 0x007D.
The TRV of Hex4Digits :: HexDigit HexDigit HexDigit HexDigit is the sequence consisting of the TRV of the first HexDigit followed by the TRV of the second HexDigit followed by the TRV of the third HexDigit followed by the TRV of the fourth HexDigit.
The TRV of HexDigits :: HexDigit is the TRV of HexDigit.
The TRV of HexDigits :: HexDigits HexDigit is the sequence consisting of TRV of HexDigits followed by TRV of HexDigit.
The TRV of a HexDigit is the SV of the SourceCharacter that is that HexDigit.
The TRV of LineContinuation :: \
LineTerminatorSequence is the sequence consisting of the code unit
value 0x005C followed by the code units of TRV of LineTerminatorSequence.
The TRV of LineTerminatorSequence :: <LF> is the code unit value 0x000A.
The TRV of LineTerminatorSequence :: <CR> is the code unit value 0x000A.
The TRV of LineTerminatorSequence :: <LS> is the code unit value 0x2028.
The TRV of LineTerminatorSequence :: <PS> is the code unit value 0x2029.
The TRV of LineTerminatorSequence :: <CR><LF> is the sequence consisting of the code unit value 0x000A.
NOTE TV excludes the code units of LineContinuation while TRV includes them. <CR><LF> and <CR> LineTerminatorSequences are normalized to <LF> for both TV and TRV. An explicit EscapeSequence is needed to include a <CR> or <CR><LF> sequence.
Certain ECMAScript statements (empty statement, let
, const
, import
, and
export
declarations, variable statement, expression statement, debugger
statement,
continue
statement, break
statement, return
statement, and throw
statement) must be terminated with semicolons. Such semicolons may always appear explicitly in the source text. For
convenience, however, such semicolons may be omitted from the source text in certain situations. These situations are
described by saying that semicolons are automatically inserted into the source code token stream in those situations.
In the following rules, “token” means the actual recognized lexical token determined using the current lexical goal symbol as described in clause 11.
There are three basic rules of semicolon insertion:
The offending token is separated from the previous token by at least one LineTerminator.
The offending token is }
.
The previous token is )
and the inserted semicolon would then be parsed as the terminating semicolon
of a do-while statement (13.7.2).
However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted automatically
if the semicolon would then be parsed as an empty statement or if that semicolon would become one of the two semicolons in
the header of a for
statement (see 13.7.4).
NOTE The following are the only restricted productions in the grammar:
++
--
continue;
continue
[no LineTerminator here] LabelIdentifier[?Yield] ;
break
;
break
[no LineTerminator here] LabelIdentifier[?Yield] ;
return
[no LineTerminator here] Expression ;
return
[no LineTerminator here] Expression[In, ?Yield] ;
throw
[no LineTerminator here] Expression[In, ?Yield] ;
=>
ConciseBody[?In]yield
[no LineTerminator here] *
AssignmentExpression[?In, Yield]yield
[no LineTerminator here] AssignmentExpression[?In, Yield]The practical effect of these restricted productions is as follows:
When a ++
or --
token is encountered where the parser would treat it as a postfix
operator, and at least one LineTerminator occurred between the preceding token and the
++
or --
token, then a semicolon is automatically inserted before the ++
or
--
token.
When a continue
, break
, return
, throw
, or yield