An introductory tutorial for learning JSX syntax in JavaScript's React framework_Basic knowledge

What is JSX?

When writing components with React, JSX syntax is usually used. At first glance, it looks like XML tags are written directly in the Javascript code. In fact, this is just syntax sugar. Every XML tag will be JSX The conversion tool converts it into pure Javascript code. Of course, you can also write it directly in pure Javascript code. However, by using JSX, the structure of the component and the relationship between the components will look clearer.

var MyComponent = React.createClass({/*...*/});
var myElement = <MyComponent someProperty={true} />;
React.render(myElement, document.body);

What will an XML tag, such as be converted into by the JSX conversion tool?

For example:

var Nav = React.createClass({/*...*/});
var app = <Nav color="blue"><Profile>click</Profile></Nav>;

will be converted to:

var Nav = React.createClass({/*...*/});
var app = React.createElement(
 Nav,
 {color:"blue"},
 React.createElement(Profile, null, "click")
);

In other words, when we write an XML tag, we are essentially calling the React.createElement method and returning a ReactElement object.

ReactElement createElement(
 string/ReactClass type,
 [object props],
 [children ...]
)

The first parameter of this method can be a string, which represents an element within the HTML standard, or an object of ReactClass type, which represents the custom component we have encapsulated before. The second parameter is an object, or a dictionary, which saves all the inherent attributes of this element (that is, the value that basically does not change after being passed in). Beginning with the third parameter, all subsequent parameters are considered child elements of the element.

JSX Converter

There are many ways to convert code with JSX syntax into pure Javascript code. For inline and HTML code or external files that have not been converted, type="text" must be added to the script tag. /jsx" and introduce the JSXTransformer.js file. However, this method is not recommended for use in a production environment. The recommended method is to convert the code before it goes online. You can use npm to install react-tools globally:

npm install -g react-tools

And use the command line tool to convert (for specific usage, please refer to jsx -h):

jsx src/ build/

If you use automation tools, such as gulp, you can use the corresponding plug-in gulp-react.

Using JS in HTML template

It is very convenient to use JS in HTML templates. You only need to use curly brackets to surround the JS code.

var names = ['Alice', 'Emily', 'Kate']; 
 
React.render( 
<div> 
{ 
names.map(function (name) { 
return <div>Hello, {name}!</div> 
}) 
} 
</div>, 
document.getElementById('example') 
); 

When compiled it becomes like this:

var names = ['Alice', 'Emily', 'Kate']; 
React.render( 
 React.createElement("div", null, names.map(function (name) { 
 return React.createElement("div", null, "Hello, ", name, "!") 
 }) ), 
 document.getElementById('example') 
); 

It should be noted that the curly braces are actually a variable output expression. JSX ultimately directly passes the content in the curly braces as the third parameter of React.createElement (passed in directly without any modification). Therefore, only one line of expression can be placed in it, and any way of writing that cannot be directly used as the third parameter is wrong, so it is wrong for you to write like this:

React.render( 
<div> 
{ 
var a = 1; 
names.map(function (name) { 
return <div>Hello, {name}!</div> 
}) 
} 
</div>, 
document.getElementById('example') 
); 

Because it is obvious that the content within the curly braces is placed directly on the third parameter, and the syntax is incorrect.

This is also wrong to write:

React.render( 
<div> 
{ 
var a = 1; 
 
} 
</div>, 
document.getElementById('example') 
); 

Because React.createElement(“div”, null, var a = 1;) is a syntax error.
Then you can also understand why the js expression in curly brackets cannot end with a semicolon.

It should be noted that if you output a JS variable in an attribute, you cannot add quotes, otherwise it will be treated as a string and not parsed.
It should look like this:

<a title={title}>链接</a>

Use HTML tags

To create an element that exists in the HTML standard, just write HTML code directly:

var myDivElement = <div className="foo" />;
React.render(myDivElement, document.body);

However, it should be noted that the two attributes of class and for, JSX syntax will eventually be converted into pure Javascript, so className and htmlFor must be used just like in Javascript DOM.

Another point is that when creating elements within the HTML standard, the JSX converter will discard those non-standard attributes. If you must add custom attributes, you need to add the data- prefix before these custom attributes.

<div data-custom-attribute="foo" />

Namespace components

For example, when developing a component, a component has multiple sub-components. If you want these sub-components to be used as attributes of its parent component, you can use it like this:

var Form = MyFormComponent;

var App = (
 <Form>
 <Form.Row>
  <Form.Label />
  <Form.Input />
 </Form.Row>
 </Form>
);

This way you only need to set the child component’s ReactClass as a property of its parent component:

var MyFormComponent = React.createClass({ ... });

MyFormComponent.Row = React.createClass({ ... });
MyFormComponent.Label = React.createClass({ ... });
MyFormComponent.Input = React.createClass({ ... });

The creation of child elements can be directly handed over to the JSX converter:

var App = (
 React.createElement(Form, null,
  React.createElement(Form.Row, null,
   React.createElement(Form.Label, null),
   React.createElement(Form.Input, null)
  )
 )
);

This feature requires version 0.11 and above

Javascript expression

To write Javascript expressions in JSX syntax, you only need to use {}, such as the following example using the ternary operator:

// Input (JSX):
var content = <Container>{window.isLoggedIn &#63; <Nav /> : <Login />}</Container>;
// Output (JS):
var content = React.createElement(
 Container,
 null,
 window.isLoggedIn &#63; React.createElement(Nav) : React.createElement(Login)
);

However, it should be noted that JSX syntax is just syntactic sugar. Behind it is calling ReactElement's constructor method React.createElement, so writing like this is not allowed:

// This JSX:
<div id={if (condition) { 'msg' }}>Hello World!</div>

// Is transformed to this JS:
React.createElement("div", {id: if (condition) { 'msg' }}, "Hello World!");

You can see obvious syntax errors in the converted Javascript code, so either use the ternary operator or write like this:

if (condition) <div id='msg'>Hello World!</div>
else <div>Hello World!</div>

传播属性（Spread Attributes）

在JSX中，可以使用...运算符，表示将一个对象的键值对与ReactElement的props属性合并，这个...运算符的实现类似于ES6 Array中的...运算符的特性。

var props = { foo: x, bar: y };
var component = <Component { ...props } />;

这样就相当于：

var component = <Component foo={x} bar={y} />

它也可以和普通的XML属性混合使用，需要同名属性，后者将覆盖前者：

var props = { foo: 'default' };
var component = <Component {...props} foo={'override'} />;
console.log(component.props.foo); // 'override'