背景
A source map is a file that maps from the transformed source to the original source,
enabling the browser to reconstruct the original source and present the reconstructed original in the debugger.
source map文件中保存了目标代码到源码之间映射关系，
使用source map，浏览器就可以在调试代码时定位到源码位置了。
本文记录一下，在webpack中与source map相关的源码学习过程。
1. 输出source map
1.1 webpack.config
webpack内置了source map生成功能，所以为了输出source map，
我们需要修改webpack.config.js，增加一些新的配置。
...
module.exports = {
devtool: 'source-map',
output: {
devtoolModuleFilenameTemplate: '[resource-path]',
...,
},
...
};
其中，devtool: 'source-map'，表示以独立文件的方式建立source map。
devtoolModuleFilenameTemplate: '[resource-path]'是一个模板，
用来配置source map文件sources数组中出现的模块路径。
1.2 修改源码
在第二篇我们的debug-webpack项目中，增加了source map配置之后，
我们修改一下 src/index.js，把它变成两行，
class A {
f() { }
}
const a = new A;
a.f();
1.3 run build
$ npm run build
我们会看到，dist/ 文件夹下有了两个文件，
$ tree -L 2 -I node_modules
debug-webpack
├── dist
│ ├── index.js
│ └── index.js.map
├── index.html
├── package.json
├── src
│ └── index.js
└── webpack.config.js
其中 index.js中的内容如下，
!function(e){var n={};function t(r){if(n[r])return n[r].exports;var o=n[r]={i:r,l:!1,exports:{}};return e[r].call(o.exports,o,o.exports,t),o.l=!0,o.exports}t.m=e,t.c=n,t.d=function(e,n,r){t.o(e,n)||Object.defineProperty(e,n,{enumerable:!0,get:r})},t.r=function(e){"undefined"!=typeof Symbol&&Symbol.toStringTag&&Object.defineProperty(e,Symbol.toStringTag,{value:"Module"}),Object.defineProperty(e,"__esModule",{value:!0})},t.t=function(e,n){if(1&n&&(e=t(e)),8&n)return e;if(4&n&&"object"==typeof e&&e&&e.__esModule)return e;var r=Object.create(null);if(t.r(r),Object.defineProperty(r,"default",{enumerable:!0,value:e}),2&n&&"string"!=typeof e)for(var o in e)t.d(r,o,function(n){return e[n]}.bind(null,o));return r},t.n=function(e){var n=e&&e.__esModule?function(){return e.default}:function(){return e};return t.d(n,"a",n),n},t.o=function(e,n){return Object.prototype.hasOwnProperty.call(e,n)},t.p="",t(t.s=0)}([function(e,n){function t(e,n){for(var t=0;t我们看到，es6的class已经被编译成了function。
index.js.map中的内容如下，
{"version":3,"sources":["webpack/bootstrap","./src/index.js"],"names":["installedModules","__webpack_require__","moduleId","exports","module","i","l","modules","call","m","c","d","name","getter","o","Object","defineProperty","enumerable","get","r","Symbol","toStringTag","value","t","mode","__esModule","ns","create","key","bind","n","object","property","prototype","hasOwnProperty","p","s","f"],"mappings":"aACA,IAAAA,KAGA,SAAAC,EAAAC,GAGA,GAAAF,EAAAE,GACA,OAAAF,EAAAE,GAAAC,QAGA,IAAAC,EAAAJ,EAAAE,IACAG,EAAAH,EACAI,GAAA,EACAH,YAUA,OANAI,EAAAL,GAAAM,KAAAJ,EAAAD,QAAAC,IAAAD,QAAAF,GAGAG,EAAAE,GAAA,EAGAF,EAAAD,QAKAF,EAAAQ,EAAAF,EAGAN,EAAAS,EAAAV,EAGAC,EAAAU,EAAA,SAAAR,EAAAS,EAAAC,GACAZ,EAAAa,EAAAX,EAAAS,IACAG,OAAAC,eAAAb,EAAAS,GAA0CK,YAAA,EAAAC,IAAAL,KAK1CZ,EAAAkB,EAAA,SAAAhB,GACA,oBAAAiB,eAAAC,aACAN,OAAAC,eAAAb,EAAAiB,OAAAC,aAAwDC,MAAA,WAExDP,OAAAC,eAAAb,EAAA,cAAiDmB,OAAA,KAQjDrB,EAAAsB,EAAA,SAAAD,EAAAE,GAEA,GADA,EAAAA,IAAAF,EAAArB,EAAAqB,IACA,EAAAE,EAAA,OAAAF,EACA,KAAAE,GAAA,iBAAAF,QAAAG,WAAA,OAAAH,EACA,IAAAI,EAAAX,OAAAY,OAAA,MAGA,GAFA1B,EAAAkB,EAAAO,GACAX,OAAAC,eAAAU,EAAA,WAAyCT,YAAA,EAAAK,UACzC,EAAAE,GAAA,iBAAAF,EAAA,QAAAM,KAAAN,EAAArB,EAAAU,EAAAe,EAAAE,EAAA,SAAAA,GAAgH,OAAAN,EAAAM,IAAqBC,KAAA,KAAAD,IACrI,OAAAF,GAIAzB,EAAA6B,EAAA,SAAA1B,GACA,IAAAS,EAAAT,KAAAqB,WACA,WAA2B,OAAArB,EAAA,SAC3B,WAAiC,OAAAA,GAEjC,OADAH,EAAAU,EAAAE,EAAA,IAAAA,GACAA,GAIAZ,EAAAa,EAAA,SAAAiB,EAAAC,GAAsD,OAAAjB,OAAAkB,UAAAC,eAAA1B,KAAAuB,EAAAC,IAGtD/B,EAAAkC,EAAA,GAIAlC,IAAAmC,EAAA,0LC9EU,iOACRC","file":"index.js","sourcesContent":[" \t// The module cache\n \tvar installedModules = {};\n\n \t// The require function\n \tfunction __webpack_require__(moduleId) {\n\n \t\t// Check if module is in cache\n \t\tif(installedModules[moduleId]) {\n \t\t\treturn installedModules[moduleId].exports;\n \t\t}\n \t\t// Create a new module (and put it into the cache)\n \t\tvar module = installedModules[moduleId] = {\n \t\t\ti: moduleId,\n \t\t\tl: false,\n \t\t\texports: {}\n \t\t};\n\n \t\t// Execute the module function\n \t\tmodules[moduleId].call(module.exports, module, module.exports, __webpack_require__);\n\n \t\t// Flag the module as loaded\n \t\tmodule.l = true;\n\n \t\t// Return the exports of the module\n \t\treturn module.exports;\n \t}\n\n\n \t// expose the modules object (__webpack_modules__)\n \t__webpack_require__.m = modules;\n\n \t// expose the module cache\n \t__webpack_require__.c = installedModules;\n\n \t// define getter function for harmony exports\n \t__webpack_require__.d = function(exports, name, getter) {\n \t\tif(!__webpack_require__.o(exports, name)) {\n \t\t\tObject.defineProperty(exports, name, { enumerable: true, get: getter });\n \t\t}\n \t};\n\n \t// define __esModule on exports\n \t__webpack_require__.r = function(exports) {\n \t\tif(typeof Symbol !== 'undefined' && Symbol.toStringTag) {\n \t\t\tObject.defineProperty(exports, Symbol.toStringTag, { value: 'Module' });\n \t\t}\n \t\tObject.defineProperty(exports, '__esModule', { value: true });\n \t};\n\n \t// create a fake namespace object\n \t// mode & 1: value is a module id, require it\n \t// mode & 2: merge all properties of value into the ns\n \t// mode & 4: return value when already ns object\n \t// mode & 8|1: behave like require\n \t__webpack_require__.t = function(value, mode) {\n \t\tif(mode & 1) value = __webpack_require__(value);\n \t\tif(mode & 8) return value;\n \t\tif((mode & 4) && typeof value === 'object' && value && value.__esModule) return value;\n \t\tvar ns = Object.create(null);\n \t\t__webpack_require__.r(ns);\n \t\tObject.defineProperty(ns, 'default', { enumerable: true, value: value });\n \t\tif(mode & 2 && typeof value != 'string') for(var key in value) __webpack_require__.d(ns, key, function(key) { return value[key]; }.bind(null, key));\n \t\treturn ns;\n \t};\n\n \t// getDefaultExport function for compatibility with non-harmony modules\n \t__webpack_require__.n = function(module) {\n \t\tvar getter = module && module.__esModule ?\n \t\t\tfunction getDefault() { return module['default']; } :\n \t\t\tfunction getModuleExports() { return module; };\n \t\t__webpack_require__.d(getter, 'a', getter);\n \t\treturn getter;\n \t};\n\n \t// Object.prototype.hasOwnProperty.call\n \t__webpack_require__.o = function(object, property) { return Object.prototype.hasOwnProperty.call(object, property); };\n\n \t// __webpack_public_path__\n \t__webpack_require__.p = \"\";\n\n\n \t// Load entry module and return exports\n \treturn __webpack_require__(__webpack_require__.s = 0);\n","class A {\n f() { }\n}\n\nconst a = new A;\na.f();"],"sourceRoot":""}
1.4 浏览器中试验一下
在工程根目录，启动一个静态资源服务器，
$ npx http-server
Starting up http-server, serving ./
Available on:
http://127.0.0.1:8080
http://xx.xx.xxx.xx:8080
Hit CTRL-C to stop the server
然后，浏览器中打开http://127.0.0.1:8080，
我们通过source map定位到了源码位置。
值得注意的是，浏览器DevTool中Network中并没有对source map文件的请求。
我们将工程中 dist/index.js.map删掉，看一下效果，
这表明source map 确实起到了作用。
2. webpack生成source map逻辑
下面我们来看一下webpack是怎样生成source map文件的。
2.1 uglifyjs-wepack-plugin
在第七篇中，我们介绍了webpack是怎样进行代码压缩的，
uglifyjs-wepack-plugin实现了compilation.hooks.optimizeChunkAssets，
然后在runner中调用了uglify-es进行压缩。
非parallel模式在Runner.js 第39行
this.boundWorkers = (options, cb) => {
try {
cb(null, minify(options));
} catch (error) {
...
}
};
minify中调用uglify-es的minify方法，位于minify.js 第156行，
const { error, map, code, warnings } = uglify.minify(
{ [file]: input },
uglifyOptions,
);
uglify-es压缩后结果最后被callback到了runner.runTasks回调中，位于index.js 第222行，
runner.runTasks(tasks, (tasksError, results) => {
...
results.forEach((data, index) => {
...
const { error, map, code, warnings, extractedComments } = data;
...
if (map) {
outputSource = new SourceMapSource(
code,
file,
JSON.parse(map), input, inputSourceMap,
);
} else {
outputSource = new RawSource(code);
}
...
uglifiedAssets.add(compilation.assets[file] = outputSource);
...
});
...
callback();
});
然后map被传入到了SourceMapSource构造函数中，
它是由webpack-sources模块导出的。
这里我们可以暂时不深究webpack-sources的细节，
只需关注outputSource怎么被webpack使用即可，
它被赋值到了compilation.assets[file]中。
compilation.assets[file] = outputSource
2.2 SourceMapDevToolPlugin
代码压缩完之后，compilation.hooks.optimizeChunkAssets hooks就执行完毕了，
重新回到了webpack主流程，位于Compilation.js 第1283行，
this.hooks.optimizeChunkAssets.callAsync(this.chunks, err => {
...
this.hooks.afterOptimizeChunkAssets.call(this.chunks);
...
});
紧接着会进行afterOptimizeChunkAssets hooks调用，
经过debug，我们看到代码跳转到了SourceMapDevToolPlugin.js 第110行
compilation.hooks.afterOptimizeChunkAssets.tap(
{
name: "SourceMapDevToolPlugin",
context: true
},
(context, chunks) => {
...
}
)
先给source map加入sources，sourceRoot，file等属性，位于SourceMapDevToolPlugin.js 第230-235行，
sourceMap.sources = moduleFilenames;
if (options.noSources) {
sourceMap.sourcesContent = undefined;
}
sourceMap.sourceRoot = options.sourceRoot || "";
sourceMap.file = file;
然后在第257行，得到了source map的文件名，
let sourceMapFile = compilation.getPath(sourceMapFilename, {
第274行，在目标文件尾部加上source map文件的引用，
assets[file] = compilation.assets[file] = new ConcatSource(
new RawSource(source),
currentSourceMappingURLComment.replace(
/\[url\]/g,
sourceMapUrl
)
);
在第282行，compilation.assets中增加了一个sourceMapFile，
assets[sourceMapFile] = compilation.assets[
sourceMapFile
] = new RawSource(sourceMapString);
它的值不再是SourceMapSource了，而是RawSource，
我们可以在调试控制台，打印一下compilation.assets['index.js.map']._value的值，
{"version":3,"sources":["webpack/bootstrap","./src/index.js"],"names":["installedModules","__webpack_require__","moduleId","exports","module","i","l","modules","call","m","c","d","name","getter","o","Object","defineProperty","enumerable","get","r","Symbol","toStringTag","value","t","mode","__esModule","ns","create","key","bind","n","object","property","prototype","hasOwnProperty","p","s","f"],"mappings":"aACA,IAAAA,KAGA,SAAAC,EAAAC,GAGA,GAAAF,EAAAE,GACA,OAAAF,EAAAE,GAAAC,QAGA,IAAAC,EAAAJ,EAAAE,IACAG,EAAAH,EACAI,GAAA,EACAH,YAUA,OANAI,EAAAL,GAAAM,KAAAJ,EAAAD,QAAAC,IAAAD,QAAAF,GAGAG,EAAAE,GAAA,EAGAF,EAAAD,QAKAF,EAAAQ,EAAAF,EAGAN,EAAAS,EAAAV,EAGAC,EAAAU,EAAA,SAAAR,EAAAS,EAAAC,GACAZ,EAAAa,EAAAX,EAAAS,IACAG,OAAAC,eAAAb,EAAAS,GAA0CK,YAAA,EAAAC,IAAAL,KAK1CZ,EAAAkB,EAAA,SAAAhB,GACA,oBAAAiB,eAAAC,aACAN,OAAAC,eAAAb,EAAAiB,OAAAC,aAAwDC,MAAA,WAExDP,OAAAC,eAAAb,EAAA,cAAiDmB,OAAA,KAQjDrB,EAAAsB,EAAA,SAAAD,EAAAE,GAEA,GADA,EAAAA,IAAAF,EAAArB,EAAAqB,IACA,EAAAE,EAAA,OAAAF,EACA,KAAAE,GAAA,iBAAAF,QAAAG,WAAA,OAAAH,EACA,IAAAI,EAAAX,OAAAY,OAAA,MAGA,GAFA1B,EAAAkB,EAAAO,GACAX,OAAAC,eAAAU,EAAA,WAAyCT,YAAA,EAAAK,UACzC,EAAAE,GAAA,iBAAAF,EAAA,QAAAM,KAAAN,EAAArB,EAAAU,EAAAe,EAAAE,EAAA,SAAAA,GAAgH,OAAAN,EAAAM,IAAqBC,KAAA,KAAAD,IACrI,OAAAF,GAIAzB,EAAA6B,EAAA,SAAA1B,GACA,IAAAS,EAAAT,KAAAqB,WACA,WAA2B,OAAArB,EAAA,SAC3B,WAAiC,OAAAA,GAEjC,OADAH,EAAAU,EAAAE,EAAA,IAAAA,GACAA,GAIAZ,EAAAa,EAAA,SAAAiB,EAAAC,GAAsD,OAAAjB,OAAAkB,UAAAC,eAAA1B,KAAAuB,EAAAC,IAGtD/B,EAAAkC,EAAA,GAIAlC,IAAAmC,EAAA,0LC9EU,iOACRC","file":"index.js","sourcesContent":[" \t// The module cache\n \tvar installedModules = {};\n\n \t// The require function\n \tfunction __webpack_require__(moduleId) {\n\n \t\t// Check if module is in cache\n \t\tif(installedModules[moduleId]) {\n \t\t\treturn installedModules[moduleId].exports;\n \t\t}\n \t\t// Create a new module (and put it into the cache)\n \t\tvar module = installedModules[moduleId] = {\n \t\t\ti: moduleId,\n \t\t\tl: false,\n \t\t\texports: {}\n \t\t};\n\n \t\t// Execute the module function\n \t\tmodules[moduleId].call(module.exports, module, module.exports, __webpack_require__);\n\n \t\t// Flag the module as loaded\n \t\tmodule.l = true;\n\n \t\t// Return the exports of the module\n \t\treturn module.exports;\n \t}\n\n\n \t// expose the modules object (__webpack_modules__)\n \t__webpack_require__.m = modules;\n\n \t// expose the module cache\n \t__webpack_require__.c = installedModules;\n\n \t// define getter function for harmony exports\n \t__webpack_require__.d = function(exports, name, getter) {\n \t\tif(!__webpack_require__.o(exports, name)) {\n \t\t\tObject.defineProperty(exports, name, { enumerable: true, get: getter });\n \t\t}\n \t};\n\n \t// define __esModule on exports\n \t__webpack_require__.r = function(exports) {\n \t\tif(typeof Symbol !== 'undefined' && Symbol.toStringTag) {\n \t\t\tObject.defineProperty(exports, Symbol.toStringTag, { value: 'Module' });\n \t\t}\n \t\tObject.defineProperty(exports, '__esModule', { value: true });\n \t};\n\n \t// create a fake namespace object\n \t// mode & 1: value is a module id, require it\n \t// mode & 2: merge all properties of value into the ns\n \t// mode & 4: return value when already ns object\n \t// mode & 8|1: behave like require\n \t__webpack_require__.t = function(value, mode) {\n \t\tif(mode & 1) value = __webpack_require__(value);\n \t\tif(mode & 8) return value;\n \t\tif((mode & 4) && typeof value === 'object' && value && value.__esModule) return value;\n \t\tvar ns = Object.create(null);\n \t\t__webpack_require__.r(ns);\n \t\tObject.defineProperty(ns, 'default', { enumerable: true, value: value });\n \t\tif(mode & 2 && typeof value != 'string') for(var key in value) __webpack_require__.d(ns, key, function(key) { return value[key]; }.bind(null, key));\n \t\treturn ns;\n \t};\n\n \t// getDefaultExport function for compatibility with non-harmony modules\n \t__webpack_require__.n = function(module) {\n \t\tvar getter = module && module.__esModule ?\n \t\t\tfunction getDefault() { return module['default']; } :\n \t\t\tfunction getModuleExports() { return module; };\n \t\t__webpack_require__.d(getter, 'a', getter);\n \t\treturn getter;\n \t};\n\n \t// Object.prototype.hasOwnProperty.call\n \t__webpack_require__.o = function(object, property) { return Object.prototype.hasOwnProperty.call(object, property); };\n\n \t// __webpack_public_path__\n \t__webpack_require__.p = \"\";\n\n\n \t// Load entry module and return exports\n \treturn __webpack_require__(__webpack_require__.s = 0);\n","class A {\n f() { }\n}\n\nconst a = new A;\na.f();"],"sourceRoot":""}
它就是最终写入index.js.map中的内容，我们可以与上文index.js.map中的内容对比来验证。
3. 写文件
source map内容生成完之后，hooks.afterOptimizeChunkAssets就会返回，
接着会依次调用，hooks.optimizeAssets，hooks.afterOptimizeAssets，hooks.afterSeal，
this.hooks.optimizeChunkAssets.callAsync(this.chunks, err => {
...
this.hooks.afterOptimizeChunkAssets.call(this.chunks);
this.hooks.optimizeAssets.callAsync(this.assets, err => {
...
this.hooks.afterOptimizeAssets.call(this.assets);
...
return this.hooks.afterSeal.callAsync(callback);
});
});
由于这些调用，都位于compilation.seal函数的调用栈中，
因此，最终回到了Compiler.js 第542行，
compilation.seal(err => {
...
});
它是由compiler.compile函数调用的，
compile(callback) {
...
this.hooks.beforeCompile.callAsync(params, err => {
...
this.hooks.make.callAsync(compilation, err => {
...
compilation.seal(err => {
...
this.hooks.afterCompile.callAsync(compilation, err => {
...
return callback(null, compilation);
});
});
});
});
}
而compiler.compile由compiler.run调用，位于Compiler.js 第268行，
this.compile(onCompiled);
3.2 emit
onCompiled函数位于Compiler.js 第211行，它调用了compiler.emitAssets，
const onCompiled = (err, compilation) => {
...
this.emitAssets(compilation, err => {
...
});
};
compiler.emitAssets会读取compilation.assets中所有内容来生成文件，
位于Compiler.js 第300行，
emitAssets(compilation, callback) {
let outputPath;
const emitFiles = err => {
if (err) return callback(err);
asyncLib.forEach(
compilation.assets,
(source, file, callback) => {
let targetFile = file;
...
const writeOut = err => {
...
const targetPath = this.outputFileSystem.join(
outputPath,
targetFile
);
...
let content = source.source();
...
this.outputFileSystem.writeFile(targetPath, content, callback);
};
if (...) {
...
this.outputFileSystem.mkdirp(
...,
writeOut
);
} else {
writeOut();
}
},
err => {
...
}
);
};
...
}
以上代码遍历了compilation.assets的所有属性file，得到了目标文件名，
然后计算得到目标文件的绝对地址，最后把内容写进去。
这里，如果开启了source map的话，
file会包含了编译后的代码文件，还有source map文件。
参考
mdn: Use a source map
wikipedia: Source mapping
Source Map Revision 3 Proposal
webpack v4.20.2
uglifyjs-webpack-plugin v1.3.0
uglify-es