In this article we will say hello to React. We'll discover a little bit of detail about its background and use cases, set up a basic React toolchain on our local computer, and create and play with a simple starter app — learning a bit about how React works in the process.
| Prerequisites: |
Familiarity with the core
HTML
,
CSS
，和
JavaScript
languages,
knowledge of the
terminal/command line
.
React uses an HTML-in-JavaScript syntax called JSX (JavaScript and XML). Familiarity with both HTML and JavaScript will help you to learn JSX, and better identify whether bugs in your application are related to JavaScript or to the more specific domain of React. |
|---|---|
| Objective: | To set up a local React development environment, create a start app, and understand the basics of how it works |
As its official tagline states, React is a library for building user interfaces. React is not a framework – it's not even exclusive to the web. It's used with other libraries to render to certain environments. For instance, React Native can be used to build mobile applications.
To build for the web, developers use React in tandem with ReactDOM . React and ReactDOM are often discussed in the same spaces as — and utilized to solve the same problems as — other true web development frameworks. When we refer to React as a "framework", we’re working with that colloquial understanding.
React's primary goal is to minimize the bugs that occur when developers are building UIs. It does this through the use of components — self-contained, logical pieces of code that describe a portion of the user interface. These components can be composed together to create a full UI, and React abstracts away much of the rendering work, leaving you to concentrate on the UI design.
Unlike the other frameworks covered in this module, React does not enforce strict rules around code conventions or file organization. This allows teams to set conventions that work best for them, and to adopt React in any way they would like to. React can handle a single button, a few pieces of an interface, or an app's entire user interface.
While React can be used for small pieces of an interface , it's not as easy to "drop into" an application as a library like jQuery, or even a framework like Vue — it is more approachable when you build your entire app with React.
In addition, many of the developer-experience benefits of a React app, such as writing interfaces with JSX, require a compilation process. Adding a compiler like Babel to a website makes the code on it run slowly, so developers often set up such tooling with a build step. React arguably has a heavy tooling requirement, but it can be learned.
This article is going to focus on the use case of using React to render the entire user interface of an application, using tooling provided by Facebook’s own create-react-app 工具。
React utilizes features of modern JavaScript for many of its patterns. Its biggest departure from JavaScript comes with the use of JSX syntax. JSX extends JavaScript's syntax so that HTML-like code can live alongside it. For example:
const heading = <h1>Mozilla Developer Network</h1>;
This heading constant is known as a
JSX expression
. React can use it to render that
<h1>
tag in our app.
Suppose we wanted to wrap our heading in a
<header>
tag, for semantic reasons? The JSX approach allows us to nest our elements within each other, just like we do with HTML:
const header = (
<header>
<h1>Mozilla Developer Network</h1>
</header>
);
注意： The parentheses in the previous snippet aren't unique to JSX, and don’t have any effect on your application. They're a signal to you (and your computer) that the multiple lines of code inside are part of the same expression. You could just as well write the header expression like this:
const header = <header>
<h1>Mozilla Developer Network</h1>
</header>
However, this looks kind of awkward, because the
<header>
tag that starts the expression is not indented to the same position as its corresponding closing tag.
Of course, your browser can't read JSX without help. When compiled (using a tool like Babel or Parcel ), our header expression would look like this:
const header = React.createElement("header", null,
React.createElement("h1", null, "Mozilla Developer Network")
);
It's
possible
to skip the compilation step and use
React.createElement()
to write your UI yourself. In doing this, however, you lose the declarative benefit of JSX, and your code becomes harder to read. Compilation is an extra step in the development process, but many developers in the React community think that the readability of JSX is worthwhile. Plus, popular tooling makes the JSX-to-JavaScript compilation part of its setup process. You don't have to configure compilation yourself unless you want to.
Because JSX is a blend of HTML and JavaScript, some developers find it intuitive. Others say that its blended nature makes it confusing. Once you're comfortable with it, however, it will allow you to build user interfaces more quickly and intuitively, and allow others to better understand your codebase at a glance.
To read more about JSX, check out the React team's JSX In Depth article.
There are many ways to use React, but we're going to use the command-line interface (CLI) tool create-react-app, as mentioned earlier, which expedites the process of developing a React application by installing some packages and creating some files for you, handling the tooling described above.
It's possible to
add React to a website without create-react-app
by copying some
<script>
elements into an HTML file, but the create-react-app CLI is a common starting point for React applications. Using it will allow you to spend more time building your app, and less time fussing with setup.
In order to use create-react-app, you need to have Node.js installed. It's recommended that you use the long-term support (LTS) version. Node includes npm (the node package manager), and npx (the node package runner).
You may also use the Yarn package manager as an alternative, but we'll assume you are using npm in this set of tutorials. See Package management basics for more information on npm and yarn.
If you're using Windows, you will need to install some software to give you parity with Unix/macOS terminal in order to use the terminal commands mentioned in this tutorial. Gitbash (which comes as part of the git for Windows toolset ) 或 Windows Subsystem for Linux ( WSL ) are both suitable. See Command line crash course for more information on these, and on terminal commands in general.
Also bear in mind that React and ReactDOM produce apps that only work on a fairly modern set of browsers — IE9+ by way of some polyfills. It is recommended that you use a modern browser like Firefox, Microsoft Edge, Safari, or Chrome when working through these tutorials.
Also, see the following for more information:
create-react-app takes one argument: the name you'd like to give your app. create-react-app uses this name to make a new directory, then creates the necessary files inside it. Make sure you
cd
to the place you'd like your app to live on your hard drive, then run the following in your terminal:
npx create-react-app moz-todo-react
这创建
moz-todo-react
directory, and does several things inside it:
注意：
if you have the yarn package manager installed, create-react-app will default to using it instead of npm. If you have both package managers installed and explicitly want to use NPM, you can add the flag
--use-npm
when you run create-react-app:
npx create-react-app moz-todo-react --use-npm
create-react-app will display a number of messages in your terminal while it works; this is normal! This might take a few minutes, so now might be a good time to go make a cup of tea.
When the process is complete,
cd
到
moz-todo-react
directory and run the command
npm start
. The scripts installed by create-react-app will start being served at a local server at localhost:3000, and open the app in a new browser tab. Your browser will display something like this:
create-react-app gives us everything we need to develop a React application. Its initial file structure looks like this:
moz-todo-react
├── README.md
├── node_modules
├── package.json
├── package-lock.json
├── .gitignore
├── public
│ ├── favicon.ico
│ ├── index.html
│ ├── logo192.png
│ ├── logo512.png
│ ├── manifest.json
│ └── robots.txt
└── src
├── App.css
├── App.js
├── App.test.js
├── index.css
├── index.js
├── logo.svg
├── reportWebVitals.js
└── setupTests.js
src
directory is where we'll spend most of our time, as it's where the source code for our application lives.
public
directory contains files that will be read by your browser while you're developing the app; the most important of these is
index.html
. React injects your code into this file so that your browser can run it. There's some other markup that helps create-react-app function, so take care not to edit it unless you know what you're doing. You very much should change the text inside the
<title>
element in this file to reflect the title of your application. Accurate page titles are important for accessibility!
public
directory will also be published when you build and deploy a production version of your app. We won’t cover deployment in this tutorial, but you should be able to use a similar solution to that described in our
Deploying our app
教程。
package.json
file contains information about our project that Node.js/npm uses to keep it organized. This file is not unique to React applications; create-react-app merely populates it. You don't need to understand this file at all to complete this tutorial, however, if you'd like to learn more about it, you can read
What is the file `package.json`? on NodeJS.org
; we also talk about it in our
Package management basics
教程。
<App/>
In React, a component is a reusable module that renders a part of our app. These parts can be big or small, but they are usually clearly defined: they serve a single, obvious purpose.
Let's open
src/App.js
, since our browser is prompting us to edit it. This file contains our first component,
App
, and a few other lines of code:
import React from 'react';
import logo from './logo.svg';
import './App.css';
function App() {
return (
<div className="App">
<header className="App-header">
<img src={logo} className="App-logo" alt="logo" />
<p>
Edit <code>src/App.js</code> and save to reload.
</p>
<a
className="App-link"
href="https://reactjs.org"
target="_blank"
rel="noopener noreferrer"
>
Learn React
</a>
</header>
</div>
);
}
export default App;
App.js
file consists of three main parts: some
import
statements at the top, the
App
component in the middle, and an
export
statement at the bottom. Most React components follow this pattern.
import
statements at the top of the file allow
App.js
to use code that has been defined elsewhere. Let's look at these statements more closely.
import React from 'react';
import logo from './logo.svg';
import './App.css';
The first statement imports the React library itself. Because React turns the JSX we write into
React.createElement()
, all React components must import the
React
module. If you skip this step, your application will produce an error.
The second statement imports a logo from
'./logo.svg'
. Note the use of
./
at the beginning of the path, and the
.svg
extension at the end — these tell us that the file is local and that it is not a JavaScript file. Indeed, the
logo.svg
file lives in our source directory.
We don't write a path or extension when importing the
React
module — this is not a local file; instead, it is listed as a dependency in our
package.json
file. Be careful of this distinction as you work through this lesson!
The third statement imports the CSS related to our App component. Note that there is no variable name and no
from
directive. This particular import syntax is not native to JavaScript module syntax — it comes from Webpack, the tool create-react-app uses to bundle all our JavaScript files together and serve them to the browser.
App
component
After the imports, we have a function named
App
. Whereas most of the JavaScript community prefers camel-case names like
helloWorld
, React components use pascal-case variable names, like
HelloWorld
, to make it clear that a given JSX element is a React component, and not a regular HTML tag. If you were to rename the
App
function to
app
, your browser would show you an error.
Let's look at
App
more closely.
function App() {
return (
<div className="App">
<header className="App-header">
<img src={logo} className="App-logo" alt="logo" />
<p>
Edit <code>src/App.js</code> and save to reload.
</p>
<a
className="App-link"
href="https://reactjs.org"
target="_blank"
rel="noopener noreferrer"
>
Learn React
</a>
</header>
</div>
);
}
App
function returns a JSX expression. This expression defines what your browser ultimately renders to the DOM.
Some elements in the expression have attributes, which are written just like in HTML, following a pattern of
attribute="value"
. On line 3, the opening
<div>
tag has a
className
attribute. This is the same as the
class
attribute in HTML, but because JSX is JavaScript, we can't use the word
class
— it's reserved, meaning JavaScript already uses it for a specific purpose and it would cause problems here in our code. A few other HTML attributes are written differently in JSX than they are in HTML too, for the same kind of reason. We'll cover them as we encounter them.
Take a moment to change the
<p>
tag on line 6 so that it reads "Hello, world!", then save your file. You'll notice that this change is immediately rendered in the development server running at
http://localhost:3000
in your browser. Now delete the
<a>
tag and save; the "Learn React" link will be gone.
您的
App
component should now look like this:
function App() {
return (
<div className="App">
<header className="App-header">
<img src={logo} className="App-logo" alt="logo" />
<p>
Hello, World!
</p>
</header>
</div>
);
}
At the very bottom of the
App.js
file, the statement
export default App
makes our
App
component available to other modules.
Let’s open
src/index.js
, because that's where the
App
component is being used. This file is the entry point for our app, and it initially looks like this:
import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import App from './App';
import * as serviceWorker from './serviceWorker';
ReactDOM.render(
<React.StrictMode>
<App />
</React.StrictMode>,
document.getElementById('root')
);
// If you want your app to work offline and load faster, you can change
// unregister() to register() below. Note this comes with some pitfalls.
// Learn more about service workers: https://bit.ly/CRA-PWA
serviceWorker.unregister();
就像
App.js
, the file starts by importing all the JS modules and other assets it needs to run.
src/index.css
holds global styles that are applied to our whole app. We can also see our
App
component imported here; it is made available for import thanks to the
export
statement at the bottom of
App.js
.
Line 7 calls React’s
ReactDOM.render()
function with two arguments:
<App />
在此情况下。
root
. If you look inside
public/index.html
, you'll see that this is a
<div>
element just inside the
<body>
.
All of this tells React that we want to render our React application with the
App
component as the root, or first component.
注意：
In JSX, React components and HTML elements must have closing slashes. Writing just
<App>
or just
<img>
will cause an error.
Service workers are interesting pieces of code that help application performance and allow features of your web applications to work offline, but they’re not in scope for this article. You can delete line 5, as well as most of the code below it.
Your final
index.js
file should look like this:
import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import App from './App';
ReactDOM.render(<App />, document.getElementById('root'));
Next, we'll use a few of our JavaScript skills to get a bit more comfortable editing components and working with data in React. We'll talk about how variables are used inside JSX, and introduce props, which are a way of passing data into a component (which can then be accessed using variables).
Back in
App.js
, let’s focus on line 9:
<img src={logo} className="App-logo" alt="logo" />
在这里，
<img />
tag's
src
attribute value is in curly braces. This is how JSX recognizes variables. React will see
{logo}
, know you are referring to the logo import on line 2 of our app, then retrieve the logo file and render it.
Let's try making a variable of our own. Before the return statement of
App
，添加
const subject = 'React';
. Your
App
component should now look like this:
function App() {
const subject = "React";
return (
<div className="App">
<header className="App-header">
<img src={logo} className="App-logo" alt="logo" />
<p>
Hello, World!
</p>
</header>
</div>
);
}
Change line 8 to use our
subject
variable instead of the word "world", like this:
function App() {
const subject = "React";
return (
<div className="App">
<header className="App-header">
<img src={logo} className="App-logo" alt="logo" />
<p>
Hello, {subject}!
</p>
</header>
</div>
);
}
When you save, your browser should display "Hello, React!" instead of "Hello, world!"
Variables are convenient, but the one we've just set doesn’t make great use of React's features. That's where props come in.
A
prop
is any data passed into a React component. React props are comparable to HTML attributes. Where HTML elements have attributes, React components have props. Props are written inside component calls, and use the same syntax as HTML attributes —
prop="value"
. In React, dataflow is unidirectional: props can only be passed from Parent components down to Child components; and props are read-only.
Let’s open
index.js
and give our
<App/>
call its first prop.
Add a prop of
subject
到
<App/>
component call, with a value of
Clarice
. When you are done, your code should look something like this:
ReactDOM.render(<App subject="Clarice" />, document.getElementById('root'));
Back in
App.js
, let's revisit the App function itself, which reads like this (with the
return
statement shortened for brevity):
function App() {
const subject = "React";
return (
// return statement
);
}
Change the signature of the
App
function so that it accepts
props
as a parameter, and delete the
subject
const. Just like any other function parameter, you can put
props
在
console.log()
to print it to your browser's console. Go ahead and do that before the
return
statement, like so:
function App(props) {
console.log(props);
return (
// return statement
);
}
Save your file and check your browser's JavaScript console. You should see something like this logged:
Object { subject: "Clarice" }
The object property
subject
corresponds to the
subject
prop we added to our
<App />
component call, and the string
Clarice
corresponds to its value. Component props in React are always collected into objects in this fashion.
Now that
subject
is one of our props, let's utilize it in
App.js
. Change the
subject
constant so that, instead of defining it as the string
React
, you are reading the value of
props.subject
. You can also delete your
console.log()
if you want.
function App(props) {
const subject = props.subject;
return (
// return statement
);
}
When you save, the app should now greet you with "Hello, Clarice!". If you return to
index.js
, edit the value of
subject
, and save, your text will change.
This brings us to the end of our initial look at React, including how to install it locally, creating a starter app, and how the basics work. In the next article, we'll start building our first proper application — a todo list. Before we do that, however, let's recap some of the things we’ve learned.
In React:
PascalCase
.
{so}
.
class
in HTML translates to
className
in JSX. Note that multi-word attributes are camel-cased.
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