Tree Shaking从字面上理解就是树摇晃。就是当我们在一个js文件中写入了多个导出的方法，但是引用时只使用了一部分，其他方法不应该被打包进来。

1.举例

文件结构

myProject
 |-dist
 |-node_modules
 |-src
+    |-util
+       |-math.js
     |-assets
        |-css
            |-index.css
        |-less
            |-index.less     
        |-sass
            |-index.scss
        |-images
            |-wali_logo.png
     |-index.html
     |-index.js
 |-package.json
 |-webpack.config.js
 |-postcss.config.js
 |-.babelrc

src/util/math.js

export const add = (x,y) =>{
    return x + y;
}

export const minus = (x,y) =>{
    return x - y;
}

src/index.js

import { add } from "./util/math";

add(1,2)

运行webpack

yarn run build

打开dist/main.js文件

...
...

/***/ "./src/util/math.js":
/*!*********************************!*\
  !*** ./src/util/math.js ***!
  \*********************************/
/*! exports provided: add, minus */
/***/ (function(module, __webpack_exports__, __webpack_require__) {

"use strict";
eval("__webpack_require__.r(__webpack_exports__);\n/* harmony export (binding) */ __webpack_require__.d(__webpack_exports__, \"add\", function() { return add; });\n/* harmony export (binding) */ __webpack_require__.d(__webpack_exports__, \"minus\", function() { return minus; });\n/**\r\n * Created by Administrator on 2019/5/28.\r\n */\nconst add = (x, y) => {\n  return x + y;\n};\nconst minus = (x, y) => {\n  return x - y;\n};//# sourceURL=[module]\n//# sourceMappingURL=data:application/json;charset=utf-8;base64,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\n//# sourceURL=webpack-internal:///./src/util/math.js\n");

/***/ })

...
...

上面这段代码就是webpack打包生成的代码，我们看到在math.js中将add,minus方法都打包进来。这样就会导致我们最终生成的文件特别大。tree shaking概念可以帮助我们将没有用的代码给摇晃掉。

2.配置webpack

package.json

{
  "name": "webpack",
  "version": "1.0.0",
  "description": "webpack测试",
+  "sideEffects":[
+	  "*.css"
+  ],
  "main": "index.js",
  "author": "wali",
  "private": true,
  "license": "MIT",
  "scripts": {
    "dev": "npx webpack-dev-server --mode=development --colors",
    "dist": "npx webpack --mode=production",
    "build": "npx webpack --mode=development --colors"
  },
  "dependencies": {
    "@babel/core": "^7.4.5",
    "@babel/plugin-transform-runtime": "^7.4.4",
    "@babel/preset-env": "^7.4.5",
    "@babel/runtime": "^7.4.5",
    "@babel/runtime-corejs2": "^7.4.5",
    "autoprefixer": "^9.5.1",
    "babel-loader": "^8.0.6",
    "clean-webpack-plugin": "^2.0.2",
    "css-loader": "^2.1.1",
    "extract-text-webpack-plugin": "^3.0.2",
    "file-loader": "^3.0.1",
    "html-loader": "^0.5.5",
    "html-webpack-plugin": "^3.2.0",
    "image-webpack-loader": "^4.6.0",
    "less": "^3.9.0",
    "less-loader": "^5.0.0",
    "lodash": "^4.17.11",
    "node-sass": "^4.12.0",
    "postcss-loader": "^3.0.0",
    "sass-loader": "^7.1.0",
    "style-loader": "^0.23.1",
    "webpack": "^4.32.0",
    "webpack-cli": "^3.3.2",
    "webpack-dev-server": "^3.4.1"
  }
}

webpack.config.js

module.exports = {
  ...
+  optimization: {
+    usedExports: true
+  }
  ...
};

这里需要特别强调下，...是说之前webpack.config.js的代码还在，只是添加了几行代码。随着功能越来越多，webpack.config.js文件的代码会越来越长，全部贴出来实在是太费篇幅了，后面小菜就都采用这种方式。

运行webpack

yarn run build

打开dist/main.js文件

...
...
/***/ "./src/util/math.js":
/*!*********************************!*\
  !*** ./src/util/math.js ***!
  \*********************************/
/*! exports provided: add, minus */
/*! exports used: add */
/***/ (function(module, __webpack_exports__, __webpack_require__) {

"use strict";
eval("/* harmony export (binding) */ __webpack_require__.d(__webpack_exports__, \"a\", function() { return add; });\n/* unused harmony export minus */\n/**\r\n * Created by Administrator on 2019/5/28.\r\n */\nconst add = (x, y) => {\n  return x + y;\n};\nconst minus = (x, y) => {\n  return x - y;\n};//# sourceURL=[module]\n//# sourceMappingURL=data:application/json;charset=utf-8;base64,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\n//# sourceURL=webpack-internal:///./src/util/math.js\n");

/***/ }),

...
...

我们发现exports used: add说明webpack已经知道在index.js文件只引用了add方法，但是没有删除，没有删除的原因是因为我们开发模式，开发模式下如果删除就会对代码行数产生影响不利定位错误，当我们把webpack模式改为生产模式，minus方法就会删除掉。

webpack.config.js

module.exports = {
  ...  
-  devtool:'cheap-eval-source-map',
+  devtool:'cheap-source-map',
  ...
};

运行生产webpack

yarn run dist

我们看到在生成环境下，就只有add方法被引入进来，而minus方法就会被摇晃掉

3.代码回滚

测试完后，我们将代码回滚下，以便后续我们继续演示。

webpack.config.js

module.exports = {
  ...  
+  devtool:'cheap-eval-source-map',
-  devtool:'cheap-source-map',
  ...
};