Tencent interviewer: Brother, you said you know Webpack, what about his principles?

foreword

Hello everyone, I'm Lin Sanxin, title Tencent Interviewer: Classmate, you said you know Webpack, what about his principles? original words that the interviewer said when the rookie was interviewing Tencent, and it was not bad, haha. The rookie must have been unable to answer at that time, and finally hung up. Today, I will simply implement the packaging principle of webpack and share it with you. Since the webpack is very complicated, today we just implement simply.

Principle diagram

• 1. First of all, you must first parse the entry file entry , convert it to AST (abstract syntax tree), and use @babel/parser
• 2. Then use @babel/traverse to find all dependent modules in the entry file
• 3. Then use @babel/core+@babel/preset-env convert the AST of the entry file to Code
• 4, the 2 found dependent module entry file, for traversal recursion repeatedly performed 1，2，3
• 5. Rewriting require function, and with 4 generated in recurrence relation with FIG output to bundle in


Code

The specific implementation principle of webpack is very complicated. Here is just a simple implementation of , so that you can have a rough understanding of how webpack works. In the code implementation process, you can own console.log and see ast，dependcies，code these specific look like, I do not show up here, see for yourself would be more sense of accomplishment, hey! !

content

config.js

This file simulates the configuration of webpack

const path = require('path')
module.exports = {
  entry: './src/index.js',
  output: {
    path: path.resolve(__dirname, './dist'),
    filename: 'main.js'
  }
}

entry file

src/index.js is the entry file

// src/index
import { age } from './aa.js'
import { name } from './hh.js'

console.log(`${name}今年${age}岁了`)

// src/aa.js
export const age = 18

// src/hh.js
console.log('我来了')
export const name = '林三心'

1. Define the Compiler class

// index.js
class Compiler {
  constructor(options) {
    // webpack 配置
    const { entry, output } = options
    // 入口
    this.entry = entry
    // 出口
    this.output = output
    // 模块
    this.modules = []
  }
  // 构建启动
  run() {}
  // 重写 require函数,输出bundle
  generate() {}
}

2. Parse the entry file and get the AST

We use @babel/parser here, which is babel7 to help us analyze the internal grammar, including es6, and return a AST abstract syntax tree

const fs = require('fs')
const parser = require('@babel/parser')
const options = require('./webpack.config')

const Parser = {
  getAst: path => {
    // 读取入口文件
    const content = fs.readFileSync(path, 'utf-8')
    // 将文件内容转为AST抽象语法树
    return parser.parse(content, {
      sourceType: 'module'
    })
  }
}

class Compiler {
  constructor(options) {
    // webpack 配置
    const { entry, output } = options
    // 入口
    this.entry = entry
    // 出口
    this.output = output
    // 模块
    this.modules = []
  }
  // 构建启动
  run() {
    const ast = Parser.getAst(this.entry)
  }
  // 重写 require函数,输出bundle
  generate() {}
}

new Compiler(options).run()

3. Find all dependent modules

Babel provides the @babel/traverse (traversal) method to maintain AST tree, which we use here to help us find the dependent modules.

const fs = require('fs')
const path = require('path')
const options = require('./webpack.config')
const parser = require('@babel/parser')
const traverse = require('@babel/traverse').default

const Parser = {
  getAst: path => {
    // 读取入口文件
    const content = fs.readFileSync(path, 'utf-8')
    // 将文件内容转为AST抽象语法树
    return parser.parse(content, {
      sourceType: 'module'
    })
  },
  getDependecies: (ast, filename) => {
    const dependecies = {}
    // 遍历所有的 import 模块,存入dependecies
    traverse(ast, {
      // 类型为 ImportDeclaration 的 AST 节点 (即为import 语句)
      ImportDeclaration({ node }) {
        const dirname = path.dirname(filename)
        // 保存依赖模块路径,之后生成依赖关系图需要用到
        const filepath = './' + path.join(dirname, node.source.value)
        dependecies[node.source.value] = filepath
      }
    })
    return dependecies
  }
}

class Compiler {
  constructor(options) {
    // webpack 配置
    const { entry, output } = options
    // 入口
    this.entry = entry
    // 出口
    this.output = output
    // 模块
    this.modules = []
  }
  // 构建启动
  run() {
    const { getAst, getDependecies } = Parser
    const ast = getAst(this.entry)
    const dependecies = getDependecies(ast, this.entry)
  }
  // 重写 require函数,输出bundle
  generate() {}
}

new Compiler(options).run()

4. Convert AST to code

Convert AST syntax tree into browser executable code, we use @babel/core and @babel/preset-env here.

const fs = require('fs')
const path = require('path')
const options = require('./webpack.config')
const parser = require('@babel/parser')
const traverse = require('@babel/traverse').default
const { transformFromAst } = require('@babel/core')

const Parser = {
  getAst: path => {
    // 读取入口文件
    const content = fs.readFileSync(path, 'utf-8')
    // 将文件内容转为AST抽象语法树
    return parser.parse(content, {
      sourceType: 'module'
    })
  },
  getDependecies: (ast, filename) => {
    const dependecies = {}
    // 遍历所有的 import 模块,存入dependecies
    traverse(ast, {
      // 类型为 ImportDeclaration 的 AST 节点 (即为import 语句)
      ImportDeclaration({ node }) {
        const dirname = path.dirname(filename)
        // 保存依赖模块路径,之后生成依赖关系图需要用到
        const filepath = './' + path.join(dirname, node.source.value)
        dependecies[node.source.value] = filepath
      }
    })
    return dependecies
  },
  getCode: ast => {
    // AST转换为code
    const { code } = transformFromAst(ast, null, {
      presets: ['@babel/preset-env']
    })
    return code
  }
}

class Compiler {
  constructor(options) {
    // webpack 配置
    const { entry, output } = options
    // 入口
    this.entry = entry
    // 出口
    this.output = output
    // 模块
    this.modules = []
  }
  // 构建启动
  run() {
    const { getAst, getDependecies, getCode } = Parser
    const ast = getAst(this.entry)
    const dependecies = getDependecies(ast, this.entry)
    const code = getCode(ast)
  }
  // 重写 require函数,输出bundle
  generate() {}
}

new Compiler(options).run()

5. Recursively resolve all dependencies and generate a dependency graph

const fs = require('fs')
const path = require('path')
const options = require('./webpack.config')
const parser = require('@babel/parser')
const traverse = require('@babel/traverse').default
const { transformFromAst } = require('@babel/core')

const Parser = {
  getAst: path => {
    // 读取入口文件
    const content = fs.readFileSync(path, 'utf-8')
    // 将文件内容转为AST抽象语法树
    return parser.parse(content, {
      sourceType: 'module'
    })
  },
  getDependecies: (ast, filename) => {
    const dependecies = {}
    // 遍历所有的 import 模块,存入dependecies
    traverse(ast, {
      // 类型为 ImportDeclaration 的 AST 节点 (即为import 语句)
      ImportDeclaration({ node }) {
        const dirname = path.dirname(filename)
        // 保存依赖模块路径,之后生成依赖关系图需要用到
        const filepath = './' + path.join(dirname, node.source.value)
        dependecies[node.source.value] = filepath
      }
    })
    return dependecies
  },
  getCode: ast => {
    // AST转换为code
    const { code } = transformFromAst(ast, null, {
      presets: ['@babel/preset-env']
    })
    return code
  }
}

class Compiler {
  constructor(options) {
    // webpack 配置
    const { entry, output } = options
    // 入口
    this.entry = entry
    // 出口
    this.output = output
    // 模块
    this.modules = []
  }
  // 构建启动
  run() {
    // 解析入口文件
    const info = this.build(this.entry)
    this.modules.push(info)
    this.modules.forEach(({ dependecies }) => {
      // 判断有依赖对象,递归解析所有依赖项
      if (dependecies) {
        for (const dependency in dependecies) {
          this.modules.push(this.build(dependecies[dependency]))
        }
      }
    })
    // 生成依赖关系图
    const dependencyGraph = this.modules.reduce(
      (graph, item) => ({
        ...graph,
        // 使用文件路径作为每个模块的唯一标识符,保存对应模块的依赖对象和文件内容
        [item.filename]: {
          dependecies: item.dependecies,
          code: item.code
        }
      }),
      {}
    )
  }
  build(filename) {
    const { getAst, getDependecies, getCode } = Parser
    const ast = getAst(filename)
    const dependecies = getDependecies(ast, filename)
    const code = getCode(ast)
    return {
      // 文件路径,可以作为每个模块的唯一标识符
      filename,
      // 依赖对象,保存着依赖模块路径
      dependecies,
      // 文件内容
      code
    }
  }
  // 重写 require函数,输出bundle
  generate() {}
}

new Compiler(options).run()

6. Rewrite the require function to output the bundle

const fs = require('fs')
const path = require('path')
const options = require('./webpack.config')
const parser = require('@babel/parser')
const traverse = require('@babel/traverse').default
const { transformFromAst } = require('@babel/core')

const Parser = {
  getAst: path => {
    // 读取入口文件
    const content = fs.readFileSync(path, 'utf-8')
    // 将文件内容转为AST抽象语法树
    return parser.parse(content, {
      sourceType: 'module'
    })
  },
  getDependecies: (ast, filename) => {
    const dependecies = {}
    // 遍历所有的 import 模块,存入dependecies
    traverse(ast, {
      // 类型为 ImportDeclaration 的 AST 节点 (即为import 语句)
      ImportDeclaration({ node }) {
        const dirname = path.dirname(filename)
        // 保存依赖模块路径,之后生成依赖关系图需要用到
        const filepath = './' + path.join(dirname, node.source.value)
        dependecies[node.source.value] = filepath
      }
    })
    return dependecies
  },
  getCode: ast => {
    // AST转换为code
    const { code } = transformFromAst(ast, null, {
      presets: ['@babel/preset-env']
    })
    return code
  }
}

class Compiler {
  constructor(options) {
    // webpack 配置
    const { entry, output } = options
    // 入口
    this.entry = entry
    // 出口
    this.output = output
    // 模块
    this.modules = []
  }
  // 构建启动
  run() {
    // 解析入口文件
    const info = this.build(this.entry)
    this.modules.push(info)
    this.modules.forEach(({ dependecies }) => {
      // 判断有依赖对象,递归解析所有依赖项
      if (dependecies) {
        for (const dependency in dependecies) {
          this.modules.push(this.build(dependecies[dependency]))
        }
      }
    })
    // 生成依赖关系图
    const dependencyGraph = this.modules.reduce(
      (graph, item) => ({
        ...graph,
        // 使用文件路径作为每个模块的唯一标识符,保存对应模块的依赖对象和文件内容
        [item.filename]: {
          dependecies: item.dependecies,
          code: item.code
        }
      }),
      {}
    )
    this.generate(dependencyGraph)
  }
  build(filename) {
    const { getAst, getDependecies, getCode } = Parser
    const ast = getAst(filename)
    const dependecies = getDependecies(ast, filename)
    const code = getCode(ast)
    return {
      // 文件路径,可以作为每个模块的唯一标识符
      filename,
      // 依赖对象,保存着依赖模块路径
      dependecies,
      // 文件内容
      code
    }
  }
  // 重写 require函数 (浏览器不能识别commonjs语法),输出bundle
  generate(code) {
    // 输出文件路径
    const filePath = path.join(this.output.path, this.output.filename)
    // 懵逼了吗? 没事,下一节我们捋一捋
    const bundle = `(function(graph){
      function require(module){
        function localRequire(relativePath){
          return require(graph[module].dependecies[relativePath])
        }
        var exports = {};
        (function(require,exports,code){
          eval(code)
        })(localRequire,exports,graph[module].code);
        return exports;
      }
      require('${this.entry}')
    })(${JSON.stringify(code)})`

    // 把文件内容写入到文件系统
    fs.writeFileSync(filePath, bundle, 'utf-8')
  }
}

new Compiler(options).run()

7. Look at the code in main

The above code is implemented, and the packaged code is written into the main.js file. Let's take a look at the code in the main.js file:

(function(graph){
      function require(module){
        function localRequire(relativePath){
          return require(graph[module].dependecies[relativePath])
        }
        var exports = {};
        (function(require,exports,code){
          eval(code)
        })(localRequire,exports,graph[module].code);
        return exports;
      }
      require('./src/index.js')
    })({
      "./src/index.js": {
          "dependecies": {
              "./aa.js": "./src\\aa.js",
              "./hh.js": "./src\\hh.js"
          },
          "code": "\"use strict\";\n\nvar _aa = require(\"./aa.js\");\n\nvar _hh = require(\"./hh.js\");\n\nconsole.log(\"\".concat(_hh.name, \"\\u4ECA\\u5E74\").concat(_aa.age, \"\\u5C81\\u4E86\"));"
      },
      "./src\\aa.js": {
          "dependecies": {},
          "code": "\"use strict\";\n\nObject.defineProperty(exports, \"__esModule\", {\n  value: true\n});\nexports.age = void 0;\nvar age = 18;\nexports.age = age;"
      },
      "./src\\hh.js": {
          "dependecies": {},
          "code": "\"use strict\";\n\nObject.defineProperty(exports, \"__esModule\", {\n  value: true\n});\nexports.name = void 0;\nconsole.log('我来了');\nvar name = '林三心';\nexports.name = name;"
      }
  })

You can execute the code of main.js and the output is:

我来了
林三心今年18岁了

Epilogue

I'm Lin Sanxin, an enthusiastic front-end rookie programmer. If you are motivated, like the front-end, and want to learn the front-end, then we can make friends and fish together haha, touch the fish group, add me, please note [Si No]