surcemap 文件里都有啥（未完）

[huruji]

推荐阅读以下文章：

https://www.mattzeunert.com/2016/02/14/how-do-source-maps-work.html

https://github.com/wayou/wayou.github.io/issues/9

https://juejin.im/post/5d0790996fb9a07f0052dbbb

读完以上文章大概可以了解到，sourcemap 的使用和相应字段的含义，如何从设计的角度逐步优化 sourcemap ，在某些情况下打断点失败的原因等

如果 sourcemap 的知识对你很重要，请阅读 sourcemap 的标准草案 https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit#

你也可以加入对应的 google group ，https://groups.google.com/forum/#!forum/mozilla.dev.js-sourcemap

以最简单的例子为例，文件内容仅仅有一个输出：

console.log(123)

通过 webpack 编译打包后，内容如下：

!function(e){var t={};function n(r){if(t[r])return t[r].exports;var o=t[r]={i:r,l:!1,exports:{}};return e[r].call(o.exports,o,o.exports,n),o.l=!0,o.exports}n.m=e,n.c=t,n.d=function(e,t,r){n.o(e,t)||Object.defineProperty(e,t,{enumerable:!0,get:r})},n.r=function(e){"undefined"!=typeof Symbol&&Symbol.toStringTag&&Object.defineProperty(e,Symbol.toStringTag,{value:"Module"}),Object.defineProperty(e,"__esModule",{value:!0})},n.t=function(e,t){if(1&t&&(e=n(e)),8&t)return e;if(4&t&&"object"==typeof e&&e&&e.__esModule)return e;var r=Object.create(null);if(n.r(r),Object.defineProperty(r,"default",{enumerable:!0,value:e}),2&t&&"string"!=typeof e)for(var o in e)n.d(r,o,function(t){return e[t]}.bind(null,o));return r},n.n=function(e){var t=e&&e.__esModule?function(){return e.default}:function(){return e};return n.d(t,"a",t),t},n.o=function(e,t){return Object.prototype.hasOwnProperty.call(e,t)},n.p="/",n(n.s=0)}([function(e,t,n){e.exports=n(1)},function(e,t){}]);
//# sourceMappingURL=test.b90aa13a0513b7f966f1.js.map

对应的 sourcemap 如下：

{"version":3,"sources":["webpack:///webpack/bootstrap"],"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"],"mappings":"aACA,IAAAA,EAAA,GAGA,SAAAC,EAAAC,GAGA,GAAAF,EAAAE,GACA,OAAAF,EAAAE,GAAAC,QAGA,IAAAC,EAAAJ,EAAAE,GAAA,CACAG,EAAAH,EACAI,GAAA,EACAH,QAAA,IAUA,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,EAAA,CAA0CK,YAAA,EAAAC,IAAAL,KAK1CZ,EAAAkB,EAAA,SAAAhB,GACA,oBAAAiB,eAAAC,aACAN,OAAAC,eAAAb,EAAAiB,OAAAC,YAAA,CAAwDC,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,IAIAlC,IAAAmC,EAAA","file":"test.b90aa13a0513b7f966f1.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"],"sourceRoot":""}

可以看到 sourcemap 的内容其实是一个 json，通过格式化工具格式化后如下：

字段含义分别是可以在标准文档中找到：

可能需要关注的是 names 和 mappings 字段，names 代表的是变量的列表，mapings 代表的就是映射表。

mapings 使用的 VLQ 编码，举例转换规则如下：

以 aACA 每个值所代表的如下：

• a: 在压缩后文件的列号
• A: 映射到 sources 文件的第几个文件
• C: 源文件的的第几行
• A: 源文件的第几列

这里可能会觉得那么似乎缺少了在压缩后的源文件的行号，这还是不能定位呀，因为一般压缩后的文件都是一行，本着不让映射文件过于庞大的原则，mapings 字段以 ; 分隔则代表一行，可以看到上面的是没有分号分隔的说明只有一行。

其中需要说明的是列号记录的是相对位置。