react的jsx语法是怎样解析的

首先我们来看看下面的代码

  import "react" from "react";
  const element = (<div>
        <div>
            <span>1</span>
            <span>2</span>
            <span>3</span>
        </div>
        <div>1</div>
        <div>2</div>
</div>)
console.log(element)

问题来了，element是如何输出上图所示的结构的？

环境配置

安装react和babel

npm i react react-dom --save
npm i @babel/core @babel/preset-env @babel/plugin-transform-react-jsx --save-dev

配置babel

{
    test: /\.(js|jsx)$/,
    include: paths.appSrc,
    loader: require.resolve('babel-loader'),
    options: {
        {
            "presets": [
                "@babel/preset-env"
            ],
            "plugins": [
                "@babel/plugin-transform-react-jsx"
            ]
        },
        cacheDirectory: true,
    }
}

@babel/plugin-transform-react-jsx做了什么？

遇到
    <div>123</div>
执行
React.createElement("div", "123");

遇到
    <div>
        <div>1</div>
        <div>2</div>
        <div>3</div>
    </div>
执行
    React.createElement("div", 
        React.createElement("div", "1"),
        React.createElement("div", "2"),
        React.createElement("div", "3")
    )
// 也就是说，用react开发的时候只要你用到了jsx语法，那么不管你有没有用到React都必须import react from "react"

写个函数来模拟它的执行过程

为了便于理解 我们把
<div>
    <div>
        <span>1</span>
        <span>2</span>
        <span>3</span>
    </div>
    <div>1</div>
    <div>2</div>
</div>
当做一棵树
let element = {
    type:"div",
    children:[{
        type:"div",
        children:[{
            type:"span",
            children:"1"
        }, {
            type:"span",
            children:"2"
        }, {
            type:"span",
            children:"3"
        }]
    }, {
        type:"div",
        children:1
    }, {
        type:"div",
        children:2
    }]
}
写一个函数对这颗树进行深度遍历

function jsxTransformNode(element, callback){
    let children = [];
    if (Array.isArray(element.children)) {  
        children = element.children.map(child => jsxTransformNode(child, callback))
    } else {
        children = [element.chidren]
    }
    return callback(element.type, ...children);
}

let nodes = jsxTransformNode(child, function ReactCreateElement(type, ...children){
    return {
        tag: type,
        children
    }
}) 

@babel/plugin-transform-react-jsx的原理

对babel不熟的话可以先看这边文章从零开始编写一个babel插件

它其实就是将

<div className="name" age="12">
    <div>1</div>
    <div>2</div>
    <div>3</div>
</div>
转化为
React.createElement(
    "div",
    {},
    React.createElement("div", {}, ...chidren),
    React.createElement("div", {}, ...chidren),
    React.createElement("div", {}, ...chidren)
)
代码块

废话不多说直接上代码，下面是我写的一个简单的babel-plugin来对jsx语法进行解析

var generator = require("@babel/generator").default
function buildAttrsCall (attribs, t){
    let properties = [];
    attribs.forEach(attr => {
        let name = attr.name.name;
        let value = attr.value;
        properties.push(t.objectProperty(t.stringLiteral(name), value))
    });
    return t.ObjectExpression(properties);
}
const createVisitor = (t) => {
    const visitor = {};
    visitor.JSXElement = {
        // 为什么是exit，因为jsx是DFS而不是BFS;
        exit(path, file){
            let openingPath = path.get("openingElement");
            let children = t.react.buildChildren(openingPath.parent);
            let tagNode = t.identifier(openingPath.node.name.name);
            // 创建React.createElement
            let createElement =  t.memberExpression(t.identifier("React"),t.identifier("createElement"));
            // 创建属性
            let attribs = buildAttrsCall(openingPath.node.attributes, t);
            // 创建React.createElement(tag, attrs, ...chidren)表达式
            let callExpr = t.callExpression(createElement, [tagNode, attribs, ...children]);
            path.replaceWith(t.inherits(callExpr, path.node));
        }
    }
    return {
        visitor,
        // 配置jsx解析器
        inherits:() => {
            return {
                manipulateOptions(opts, parserOpts) {
                    parserOpts.plugins.push("jsx");
                }
            };

        }
    }
}
module.exports = function(babel){
    const t = babel.types;
    return createVisitor(t);
}

参考 React面试题详细解答

1. 创建tagNode变量
2. 创建React.createElement表达式
3. 创建attribs对象
4. 创建React.createElement("div", {}, ...children)表达式
5. 最后替换node

效果如下

源代码如下

const a = <div className="name" age="12">
    <div>1</div>
    <div>2</div>
    <div>3</div>
</div>;

编译之后

var a = React.createElement(div, {
  "className": "name",
  "age": "12"
}, React.createElement(div, {}, "1"), React.createElement(div, {}, "2"), React.createElement(div, {}, "3"));
console.log(a);