Going from Local State to a Redux Store in a React App
When choosing between local state and a redux store there is one main factor you want to take into account: how many components need access to the state. Just one, use local state. More than one, you might want to use Redux. What is Redux? Well in this post I'm going to define the basic components of Redux and Thunk while explaining how to change from locally defined state to using a Redux store.
In the following image I have a fetch set up as local state.
Redux is a JavaScript library that stores all of our data in a global state so that we can access it from across all components in our application.
In order to use Redux you need to run either
npx install redux
npx install react-redux
npx install thunk
or
yarn install redux
yarn install react-redux
yarn install thunk
in your terminal.
NOTE: The redux library is not exclusive to React - it can be used with other JS frameworks.
Next you need to set up your index.js file like this:
You are importing a provider, reducer, createStore, thunk and middleware. Then you are making a store with middleware using thunk and line 13 allows use of window's devtools. Lines 18 and 22 are wrapped around App which makes it so that redux is used for state. Our provider on line 18 is for connecting our react app to our redux store.
The redux store is an object that stores the global state in our app.
Next you want to make a redux folder in your src folder in which to store all of your redux files. Inside the redux folder you want to add an actions folder and a reducers folder and make a reducers file.
A reducer is just a function that returns state. I am using a combined reducer here. It turns out that redux lets us combine multiple reducers into one that can be passed into createStore by using a helper function named combineReducers. This way I can put more than one reducer in my blogReducer.js file.
Examining the file blogReducer.js will gain the information that the state is set to an empty array and there is something called an action.type and an action.payload. An action is a JavaScript object that can be an asyc function. Redux documentation states that 'you can think of an action as an event that describes something that happened in the application.' An action has a type and a payload. Action.type is a string and should be all caps. Action.payload will be other fields with additional informaion in them.
const add1Action = {
type: 'ADD_ONE',
payload: + 1
}
Due to the asynchronous nature of react, if your action is a function, then you need thunk. Thunk allows asynchronous functions to return a dispatch action in the form of a function (line 3 and 4 in above image). Otherwise it returns an action object.
If you follow the logic in the image below, first the componentDidMount() fires which you can see because of console.log('A'). Then fetchBlogs() fires which you can see becase of console.log('B'). Then console.log('C') fires before console.log('D') does. This is because console.log('D') cannot fire until the 2nd promise is returned in the fetch.
In order to connect our components to the store, we need to import connect and use mapStateToProps and mapDispatchToProps. Destructuring is the process of breaking a structure. In the context of programming, the structures are the data structures, and destructuring these data structures means unpacking individual values from the data structure. On the left side, in BlogContainer.js, the connect statment has mapStateToProps and mapDispatchToProps destructured. They are in a longer format on the right side to show the other way they can be called.
I defined the major terms associated with redux and explained the basics about how to set up a redux store. Obviously the time when you want redux is in a significantly larger app than this, but this gives the basic idea. Good luck and happy coding.


{% extends "base.html" %} {% block title %} 免费小说网 {% endblock %} {% block novels %} {% for novel in novels %}


Why Read This Book?
Because it does so much, Emacs has a reputation for being extremely
complicated. We don't think that's
warranted; we teach you Emacs from the ground up, covering first the
basics and then some of the more advanced features.
In this book, we have tried to reach as broad an audience as
possible: from casual users to professional writers and web authors
to programmers to system administrators. No matter what you do with
Emacs, you will find it's easy to learn; after one
or two sessions, you'll know the basics of editing
any file. After you learn the basics, you can go on to learn about
more advanced topics that provide the real benefits of using Emacs.
These include:
Using multiple windows and buffers so you can work on several files
at once Customizing keyboard commands Tailoring Emacs to fit your work style Making Emacs your work environment where you can do all your everyday
tasks, such as organizing files, compiling programs, and issuing
shell commands Creating macros to streamline repetitive tasks Using Emacs to support programming in many languages (including C,
C++, Lisp, Java, and Perl) Formatting files with various markup languages, such as HTML and XML Using word abbreviations to avoid spelling out long phrases or to
correct common misspellings
Of course, many of the topics may not apply to you; some topics may
be appropriate for a second reading but not for the first. Toward the
end of the preface, we'll sketch several different
ways to approach the book, depending on your interests and
experience.
|


bootstrap-drawer
Basics
Home / Basic Usage
Position / Colors
data-toggle / Javascript
Using Less
Other Links
GitHub Repo
npm
@clineamb
bootstrap-drawer
bootstrap-drawer is a Bootstrap add-on that gives you an off-canvas/drawer element to your page or container! Check it out by clicking the toggle in the top-left corner!


SongBox is built and shared by Will Worth. All bug reports, feature suggestions, and comments gratefully received.
SongBox is designed to store your lyrics, notes, and musical ideas.
Organise your original material with supporting notes, as well as all those covers you´ve been trying to keep track of.
SongBox is open source, so you can host it on your local machine, self-hosted wordpress site(or whatever), and tweak it as you please.


Accessing deeply nested data
Getting an Uncaught TypeError: Cannot read property
error in JavaScript has happened to me too many times. The error has even been ranked the most frequent JavaScript error. Especially when working with external data sources
this can be problem.
Consider the following example from the harmonie project, which parses agricultural field data from various input files (mostly XML
):
let applicationYear
// try to access the application year from the farm subsidy form
try {
applicationYear = data['fsv:FSV']['fsv:FSVHeader']['commons:Antragsjahr']
} catch (e) {
// no application year found -> go on as it is optional anyway
}
The try...catch
block will now capture the error thrown when a property cannot be read. We could see this as ‘defensive code’, as we can’t trust the input data and have to consider missing properties in the nested tree. Instead of repeating the try..catch
block for every variable I have been using the following function in the past (e.g. in the ELAN-Paser):
function getSafe(func) {
try {
return func()
} catch (e) {
return ''
}
}
const applicationYear = getSafe(() => data['fsv:FSV']['fsv:FSVHeader']['commons:Antragsjahr'])
// applicationYear is either the correct applicationYear from the data object OR undefined
This shortens the amount of code to write every time you are unsure whether a property exists in an object or not. While it is not much extra code, it is still repetitive and the function needs to be imported/defined in every new project. With the JavaScript standard ES2020, we don’t need this anymore.
A better way with optional chaining introduced with ES2020
With the optional chaining feature introduced in ES2020, we don’t need to define a getSafe
function first, but can rather rely on native methods:
const applicationYear = data['fsv:FSV']?.['fsv:FSVHeader']?.['commons:Antragsjahr']
// applicationYear is either the correct applicationYear from the data object OR undefined
Awesome, isn’t it? As of today, all up to date versions of modern browsers (Chrome,Firefox,Edge,Safari,Opera) support optional chaining. If you need support for older browsers, there also is a babel plugin as well.
Uncaught TypeErrors that cannot be solved by optional chaining
Another classic example for getting the error is by misspelling a property name in a nested object:
const field = {
properties: {
cropCode: 415
}
}
// misspelling key 'properties'
const cropCode = field.propetries.cropCode
// will thrown: Uncaught TypeError...
Obviously, this cannot be solved by optional chaining, as the information is not optional. Using TypeScript would have saved me from this error many times, however as of right now I’m just too lazy to set it up and annotate all of my code properly. Maybe in the not so distant future I will give it a try…