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When your source code has gone through transformations, debugging becomes a problem. When debugging in a browser, how to tell where the original code is? Source maps solve this problem by providing a mapping between the original and the transformed source code. In addition to source compiling to JavaScript, this works for styling as well. 当你的源代码完成转换后，调试就成了一个问题。在浏览器中调试时，如何确认代码中的原始位置呢？Source maps 通过建立源码与转换后的代码之间的映射关系来解决这个问题。除了可以用在编译后的 Javascript 中，还可以用在样式中。
One approach is to skip source maps during development and rely on browser support of language features. If you use ES2015 without any extensions and develop using a modern browser, this can work. The advantage of doing this is that you avoid all the problems related to source maps while gaining better performance. 在开发中不使用源码映射的方法是依赖于浏览器对语言特性的支持。如果你只使用 ES015 而不使用任何扩展并且在开发中使用现代浏览器，这是可以实现的。这样做的好处就是为了获取更好的性能时可以避免由于源码映射导致的性能下降问题。
If you are using webpack 4 and the new mode option, the tool will generate source maps automatically for you in development mode. Production usage requires attention, though. 如果你在使用 Webpack4 并且使用 mode 选项，在 development 模式下，Webpack 会自动生成源码映射关系。但是需要注意生产环境的使用。
T> If you want to understand the ideas behind source maps in greater detail, read Ryan Seddon’s introduction to the topic. 如果想深入了解 source map 的机制，可以读 Ryan Seddon 的介绍.
T> To see how webpack handles source maps, see source-map-visualization by the author of the tool.T> 想了解 webpack 如何处理源码映射, 查看工具作者写的 source-map-visualization .
Inline Source Maps and Separate Source Maps(内联源码映射和分离源码映射)
Webpack can generate both inline or separate source map files. The inline ones are valuable during development due to better performance while the separate ones are handy for production use as it keeps the bundle size small. In this case, loading source maps is optional.Webpack 即可以生成内联源码映射也可以生成分离源码映射。内联源码映射在开发时非常有用，而为了追求生产环境中更好的性能，分离源码映射通过保持 Bundle 的文件更小而更加方便。这种情况下，加载源码映射是选择性的。
It’s possible you don’t want to generate a source map for your production bundle as this makes it effortless to inspect your application. By disabling source maps, you are performing a sort of obfuscation. Whether or not you want to enable source maps for production, they are handy for staging. Skipping source maps speeds up your build as generating source maps at the best quality can be a complicated operation. 你一定不想在生产环境中生成源码映射，因为它能用来非常容易的分析你的程序。通过禁用源码映射，你要执行一系列混淆。无论你想不想启用源码映射，但是它们对于演示来说非常方便。跳过源码映射可以加快你的构建速度，而生成源码映射可以提升质量，这可能成为一个复杂的操作。
Hidden source maps give stack trace information only. You can connect them with a monitoring service to get traces as the application crashes allowing you to fix the problematic situations. While this isn’t ideal, it’s better to know about possible problems than not. 隐藏源码映射 只能提供堆栈跟踪信息。在程序崩溃的时候，可以让你把他们连接到监视服务来获取跟踪信息来解决有问题的情况。虽然这样并不理想，但是知道可能的问题总比不知道要好。
T> It’s a good idea to study the documentation of the loaders you are using to see loader specific tips. For example, with TypeScript, you have to set a particular flag to make it work as you expect. 学习你在使用的加载器的具体提示是个好主意。例如，在 Typescript 中，你得设置一个标志才能让它按照预想的方式工作。
Enabling Source Maps(启用源码映射)
Webpack provides two ways to enable source maps. There’s a devtool shortcut field. You can also find two plugins that give more options to tweak. The plugins are going to be discussed briefly at the end of this chapter. Beyond webpack, you also have to enable support for source maps at the browsers you are using for development.Webpack 有两种方式来启用源码映射。有一个字段 devtool。你也能找到两个插件，他们能提供更多的选项做进一步配置。这些插件将在本文最后做个简单的介绍。除了 Webpack，我们还得在开发使用的浏览器中启用源码映射支持。
Enabling Source Maps in Webpack(启用 Webpack 中的源码映射)
To get started, you can wrap the core idea within a configuration part. You can convert this to use the plugins later if you want: 在开始前，你可以把这个核心思想封装在一个配置部分中。如果愿意，你可以稍后将其转换成插件：
webpack.parts.js
exports.generateSourceMaps = ({type}) => ({
devtool: type,
});
Webpack supports a wide variety of source map types. These vary based on quality and build speed. For now, you enable source-map for production and let webpack use the default for development. Set it up as follows:Webpack 支持很多种源码映射类型。他们的不同在于质量和构建速度。现在，为生产环境启用 source-map 并且让 webpack 对开发环境使用默认配置。按如下方式配置：
webpack.config.js
const productionConfig = merge([
leanpub-start-insert
parts.generateSourceMaps({type: “source-map”}),
leanpub-end-insert
…
]);
source-map is the slowest and highest quality option of them all, but that’s fine for a production build.source-map 是最慢但是质量却最高的，这对生产环境构建很适合。
{pagebreak}
If you build the project now (npm run build), you should see source maps in the output: 如果你现在构建项目（npm run build），就能在输出中看到源码映射：
Hash: b59445cb2b9ae4cea11b
Version: webpack 4.1.1
Time: 1347ms
Built at: 3/16/2018 4:58:14 PM
Asset Size Chunks Chunk Names
main.js 838 bytes 0 [emitted] main
main.css 3.49 KiB 0 [emitted] main
main.js.map 3.75 KiB 0 [emitted] main
main.css.map 85 bytes 0 [emitted] main
index.html 220 bytes [emitted]
Entrypoint main = main.js main.css main.js.map main.css.map
…
Take a good look at those .map files. That’s where the mapping between the generated and the source happens. During development, it writes the mapping information in the bundle. 仔细查看这些 .map 文件。这就是保存源码和编译后的代码的映射关系的地方。在开发的时候，它把映射信息直接写到 Bundle 中。
Enabling Source Maps in Browsers(在浏览器中启用源码映射)
To use source maps within a browser, you have to enable source maps explicitly as per browser-specific instructions: 要想在浏览器中使用源码映射，你得根据不同浏览器的具体介绍准确启用源码映射。
Chrome. Sometimes source maps will not update in Chrome inspector. For now, the temporary fix is to force the inspector to reload itself by using alt-r.
Firefox
IE Edge
Safari
W> If you want to use breakpoints (i.e., a debugger; statement or ones set through the browser), the eval-based options won’t work in Chrome! 如果你想用断点（例如，一个 debugger 语句或者通过浏览器设置），在 chrome 中基于 eval 的选项不起作用。
Source Map Types Supported by Webpack(webpack 支持的源码映射类型)
Source map types supported by webpack can be split into two categories:Webpack 支持的源码映射可以分为两类：
Inline source maps add the mapping data directly to the generated files.
Inline 源码映射直接将映射数据添加到生成文件中.
Separate source maps emit the mapping data to separate source map files and link the source to them using a comment. Hidden source maps omit the comment on purpose.
Separate 源码映射将映射数据生成到独立的源码映射文件中并通过注释将他们的源码关联起来. Hidden source maps 故意省略注释。
Thanks to their speed, inline source maps are ideal for development. Given they make the bundles big, separate source maps are the preferred solution for production. Separate source maps work during development as well if the performance overhead is acceptable. 内联源码映射由于速度快非常适合开发使用。由于他们使 Bundle 变大，分离的源码映射更受到生产环境的青睐。如果性能开销可以接受，分离源码映射也可以用在开发中。
Inline Source Map Types(内联源码映射类型)
Webpack provides multiple inline source map variants. Often eval is the starting point and webpack issue #2145 recommends cheap-module-eval-source-map as it’s a good compromise between speed and quality while working reliably in Chrome and Firefox browsers.Webpack 提供多种内联源码映射变体。eval 通常作为开始点，在 webpack issue #2145 中推荐使用 cheap-module-eval-source-map 由于其很好的平衡了速度和质量并且在 Firefox 和 Chrome 中运行稳定。
To get a better idea of the available options, they are listed below while providing a small example for each. The source code contains only a single console.log(‘Hello world’) and webpack.NamedModulesPlugin is used to keep the output easier to understand. In practice, you would see a lot more code to handle the mapping. 为了更好的离解可用的选项，下面列举并使用短小的例子进行说明。源码中只包含一句 console.log(‘Hello world’) 并且 webpack.NamedModulesPlugin 保证输出更易于理解。在实际中，你将会看到更多的代码来处理映射关系。
devtool: “eval”
eval generates code in which each module is wrapped within an eval function:eval 生成代码，每个模块都封装在一个 eval 函数中:
webpackJsonp([1, 2], {
“./src/index.js”: function(module, exports) {
eval(“console.log(‘Hello world’);\n\n//////////////////\n// WEBPACK FOOTER\n// ./src/index.js\n// module id = ./src/index.js\n// module chunks = 1\n\n//# sourceURL=webpack:///./src/index.js?”)
}
}, [“./src/index.js”]);
devtool: “cheap-eval-source-map”
cheap-eval-source-map goes a step further and it includes base64 encoded version of the code as a data url. The result contains only line data while losing column mappings.cheap-eval-source-map 更进一步，它包含 base64 编码版本的代码作为数据的 URL。结果中只包含行数据而没有列的映射数据。
webpackJsonp([1, 2], {
“./src/index.js”: function(module, exports) {
eval(“console.log(‘Hello world’);//# sourceMappingURL=data:application/json;charset=utf-8;base64,eyJ2ZXJzaW9uIjozLCJmaWxlIjoiLi9hcHAvaW5kZXguanMuanMiLCJzb3VyY2VzIjpbIndlYnBhY2s6Ly8vLi9hcHAvaW5kZXguanM/MGUwNCJdLCJzb3VyY2VzQ29udGVudCI6WyJjb25zb2xlLmxvZygnSGVsbG8gd29ybGQnKTtcblxuXG4vLy8vLy8vLy8vLy8vLy8vLy9cbi8vIFdFQlBBQ0sgRk9PVEVSXG4vLyAuL2FwcC9pbmRleC5qc1xuLy8gbW9kdWxlIGlkID0gLi9hcHAvaW5kZXguanNcbi8vIG1vZHVsZSBjaHVua3MgPSAxIl0sIm1hcHBpbmdzIjoiQUFBQSIsInNvdXJjZVJvb3QiOiIifQ==”)
}
}, [“./src/index.js”]);
{pagebreak}
If you decode that base64 string, you get output containing the mapping: 如果解码 base64 的字符串，就能得到包含映射关系的输出：
{
“file”: “./src/index.js”,
“mappings”: “AAAA”,
“sourceRoot”: “”,
“sources”: [
“webpack:///./src/index.js?0e04”
],
“sourcesContent”: [
“console.log(‘Hello world’);\n\n\n//////////////////\n// WEBPACK FOOTER\n// ./src/index.js\n// module id = ./src/index.js\n// module chunks = 1”
],
“version”: 3
}
devtool: “cheap-module-eval-source-map”
cheap-module-eval-source-map is the same idea, except with higher quality and lower performance:cheap-module-eval-source-map 也是相同的思路，只是质量更高，但是性能更低：
webpackJsonp([1, 2], {
“./src/index.js”: function(module, exports) {
eval(“console.log(‘Hello world’);//# sourceMappingURL=data:application/json;charset=utf-8;base64,eyJ2ZXJzaW9uIjozLCJmaWxlIjoiLi9hcHAvaW5kZXguanMuanMiLCJzb3VyY2VzIjpbIndlYnBhY2s6Ly8vYXBwL2luZGV4LmpzPzIwMTgiXSwic291cmNlc0NvbnRlbnQiOlsiY29uc29sZS5sb2coJ0hlbGxvIHdvcmxkJyk7XG5cblxuLy8gV0VCUEFDSyBGT09URVIgLy9cbi8vIGFwcC9pbmRleC5qcyJdLCJtYXBwaW5ncyI6IkFBQUEiLCJzb3VyY2VSb290IjoiIn0=”)
}
}, [“./src/index.js”]);
{pagebreak}
Again, decoding the data reveals more: 再一次，解码数据揭示更多信息：
{
“file”: “./src/index.js”,
“mappings”: “AAAA”,
“sourceRoot”: “”,
“sources”: [
“webpack:///src/index.js?2018”
],
“sourcesContent”: [
“console.log(‘Hello world’);\n\n\n// WEBPACK FOOTER //\n// src/index.js”
],
“version”: 3
}
In this particular case, the difference between the options is minimal. 在这个特定的情况中，选项之间的差异是很小的。
devtool: “eval-source-map”
eval-source-map is the highest quality option of the inline options. It’s also the slowest one as it emits the most data:eval-source-map 是内联选项中质量最高的选项。由于它产生最多的数据所以也是最慢的。
webpackJsonp([1, 2], {
“./src/index.js”: function(module, exports) {
eval(“console.log(‘Hello world’);//# sourceMappingURL=data:application/json;charset=utf-8;base64,eyJ2ZXJzaW9uIjozLCJzb3VyY2VzIjpbIndlYnBhY2s6Ly8vLi9hcHAvaW5kZXguanM/ZGFkYyJdLCJuYW1lcyI6WyJjb25zb2xlIiwibG9nIl0sIm1hcHBpbmdzIjoiQUFBQUEsUUFBUUMsR0FBUixDQUFZLGFBQVoiLCJmaWxlIjoiLi9hcHAvaW5kZXguanMuanMiLCJzb3VyY2VzQ29udGVudCI6WyJjb25zb2xlLmxvZygnSGVsbG8gd29ybGQnKTtcblxuXG4vLyBXRUJQQUNLIEZPT1RFUiAvL1xuLy8gLi9hcHAvaW5kZXguanMiXSwic291cmNlUm9vdCI6IiJ9”)
}
}, [“./src/index.js”]);
{pagebreak}
This time around there’s more mapping data available for the browser: 这回有更多的映射数据可供浏览器使用：
{
“file”: “./src/index.js”,
“mappings”: “AAAAA,QAAQC,GAAR,CAAY,aAAZ”,
“names”: [
“console”,
“log”
],
“sourceRoot”: “”,
“sources”: [
“webpack:///./src/index.js?dadc”
],
“sourcesContent”: [
“console.log(‘Hello world’);\n\n\n// WEBPACK FOOTER //\n// ./src/index.js”
],
“version”: 3
}
Separate Source Map Types（分离式源码映射类型）
Webpack can also generate production usage friendly source maps. These end up in separate files ending with .map extension and are loaded by the browser only when required. This way your users get good performance while it’s easier for you to debug the application.Webpack 还能生成适用于生产环境的源码映射。它们最终存入以.map 为扩展名的单独文件中，并且只在需要的情况下由浏览器加载。用户可以获得更好的性能，同时他也可以 Debug 程序。
source-map is a reasonable default here. Even though it takes longer to generate the source maps this way, you get the best quality. If you don’t care about production source maps, you can skip the setting there and get better performance in return.source-map 是这里理所当然的默认值。尽管按这种方式生成代码映射会耗费更长时间，但是你能获得更好的性能。如果你不介意生产环境的代码映射，可以跳过此步来获取更好的性能。
{pagebreak}
devtool: “cheap-source-map”
cheap-source-map is similar to the cheap options above. The result is going to miss column mappings. Also, source maps from loaders, such as css-loader, are not going to be used.cheap-source-map 和上面介绍的简易选项一样。结果就是丢失列映射关系。此外，来自加载器的源码映射（如 css-loader）将不能使用。
Examining the .map file reveals the following output in this case: 在这个例子中，查看.map 文件可以得到以下输出:
{
“file”: “main.9aff3b1eced1f089ef18.js”,
“mappings”: “AAAA”,
“sourceRoot”: “”,
“sources”: [
“webpack:///main.9aff3b1eced1f089ef18.js”
],
“sourcesContent”: [
“webpackJsonp([1,2],{\”./src/index.js\”:function(o,n){console.log(\”Hello world\”)}},[\”./src/index.js\”]);\n\n\n// WEBPACK FOOTER //\n// main.9aff3b1eced1f089ef18.js”
],
“version”: 3
}
The source contains //# sourceMappingURL=main.9a…18.js.map kind of comment at its end to map to this file. 源码结尾中包含 //# sourceMappingURL=main.9a…18.js.map 这样的注释来指向分离的映射文件。
{pagebreak}
devtool: “cheap-module-source-map”
cheap-module-source-map is the same as previous except source maps from loaders are simplified to a single mapping per line. It yields the following output in this case: 除了加载器的源映射被简化为每行一个映射之外，cheap-module-source-map 与前面的映射是相同的。在这种情况下，它产生如下输出:
{
“file”: “main.9aff3b1eced1f089ef18.js”,
“mappings”: “AAAA”,
“sourceRoot”: “”,
“sources”: [
“webpack:///main.9aff3b1eced1f089ef18.js”
],
“version”: 3
}
W> cheap-module-source-map is currently broken if minification is used and this is an excellent reason to avoid the option for now.W> 如果使用了 minification cheap-module-source-map 就会遭到破坏，这也成为不使用这个选项的绝佳理由.
devtool: “hidden-source-map”
hidden-source-map is the same as source-map except it doesn’t write references to the source maps to the source files. If you don’t want to expose source maps to development tools directly while you wish proper stack traces, this is handy.hidden-source-map 与 source-map 一样，只不过它不会将源码映射的引用写入源文件中。如果您不想直接将源映射公开给开发工具，而希望使用适当的堆栈跟踪，那么这是非常方便的。
devtool: “nosources-source-map”
nosources-source-map creates a source map without sourcesContent in it. You still get stack traces, though. The option is useful if you don’t want to expose your source code to the client.nosources-source-map 创建的源码映射里不包含代码内容。但是你还是能得到堆栈跟踪信息。如果不想暴露源码给用户这个选项很有用。
T> The official documentation contains more information about devtool options.T> The official documentation 包含更多关于 devtool 选项的信息.
{pagebreak}
devtool: “source-map”
source-map provides the best quality with the complete result, but it’s also the slowest option. The output reflects this:source-map 用完整的结果提供最好的质量，但是它也是最慢的选项。输出如下所示：
{
“file”: “main.9aff3b1eced1f089ef18.js”,
“mappings”: “AAAAA,cAAc,EAAE,IAEVC,iBACA,SAAUC,EAAQC,GCHxBC,QAAQC,IAAI,kBDST”,
“names”: [
“webpackJsonp”,
“./src/index.js”,
“module”,
“exports”,
“console”,
“log”
],
“sourceRoot”: “”,
“sources”: [
“webpack:///main.9aff3b1eced1f089ef18.js”,
“webpack:///./src/index.js”
],
“sourcesContent”: [
“webpackJsonp([1,2],{\n\n/***/ \”./src/index.js\”:\n/***/ (function(module, exports) {\n\nconsole.log(‘Hello world’);\n\n/***/ })\n\n},[\”./src/index.js\”]);\n\n\n// WEBPACK FOOTER //\n// main.9aff3b1eced1f089ef18.js”,
“console.log(‘Hello world’);\n\n\n// WEBPACK FOOTER //\n// ./src/index.js”
],
“version”: 3
}
{pagebreak}
Other Source Map Options（其他源码映射选项）
There are a couple of other options that affect source map generation: 还有其他一些选项影响源码映射的生成：
{
output: {
// Modify the name of the generated source map file.
// You can use [file], [id], and [hash] replacements here.
// The default option is enough for most use cases.
sourceMapFilename: ‘[file].map’, // Default
// This is the source map filename template. It’s default
// format depends on the devtool option used. You don’t
// need to modify this often.
devtoolModuleFilenameTemplate:
‘webpack:///[resource-path]?[loaders]’
},
}
T> The official documentation digs into output specifics.T> 官方文档 深入研究了 output 的细节 .
W> If you are using UglifyJsPlugin and still want source maps, you need to enable sourceMap: true for the plugin. Otherwise, the result isn’t what you expect because UglifyJS will perform a further transformation of the code, breaking the mapping. The same has to be done with other plugins and loaders performing changes. css-loader and related loaders are a good example.W> 如果你在使用 UglifyJsPlugin，并且想使用源码映射，你需要在插件中启用 sourcemap:true。否则，结果与你的预期会不一致，因为 UglifyJS 会执行更深入的代码转换从而破坏了映射。在其他的插件和加载器中也需要做相应的修改。css-loader 和相关的加载器就是很好的例子。
SourceMapDevToolPlugin and EvalSourceMapDevToolPlugin
If you want more control over source map generation, it’s possible to use the SourceMapDevToolPlugin or EvalSourceMapDevToolPlugin instead. The latter is a more limited alternative, and as stated by its name, it’s handy for generating eval based source maps. 如果想对源码映射进行更多的控制，你可以使用 SourceMapDevToolPlugin 或者 EvalSourceMapDevToolPlugin 来代替. 后者是一种更有限的选择，正如其名称所述，它对于生成基于 eval 的源码映射非常方便。
Both plugins can allow more granular control over which portions of the code you want to generate source maps for, while also having strict control over the result with SourceMapDevToolPlugin. Using either plugin allows you to skip the devtool option altogether. 这两个插件都允许更细粒度地控制要为哪部分代码生成源码映射, 同时还可以使用“SourceMapDevToolPlugin”严格控制结果。使用任何一个插件都可以跳过“devtool”选项。
Given webpack matches only .js and .css files by default for source maps, you can use SourceMapDevToolPlugin to overcome this issue. This can be achieved by passing a test pattern like /\.(js|jsx|css)($|\?)/i. 考虑到 webpack 生成源码匹配时默认只匹配 .js 和 .css 文件，你可以使用 SouceMapDevToolPlugin 来解决这个问题。可以通过传入一个 像 /\.(js|jsx|css)($|\?)/i`test 这样的 test` 模式来解决。
EvalSourceMapDevToolPlugin accepts only module and lineToLine options as described above. Therefore it can be considered as an alias to devtool: “eval” while allowing a notch more flexibility. 如上所述，EvalSourceMapDevToolPlugin 只接受 module 和 lineToLine 选项。因此可以把它看成是 devtool: “eval” 的别称，只不过是配置更富弹性。
Changing Source Map Prefix(修改源码映射前缀)
You can prefix a source map option with a pragma character that gets injected into the source map reference. Webpack uses # by default that is supported by modern browsers, so you don’t have to set it. 您可以在源码映射选项前面加上一个编译指示字符，该字符被注入到源码映射引用中。Webpack 默认使用现代浏览器都支持的 #, 所以不用特地去设置它。
To override this, you have to prefix your source map option with it (e.g., @source-map). After the change, you should see //@ kind of reference to the source map over //# in your JavaScript files assuming a separate source map type was used. 要替换它的话，你需要把它添加到源码映射选项的前面（例如：@source-map）. 修改之后，如果是正在使用分离映射文件，你应该能在源码映射中看到 //@这样的引用而不是 //#。
Using Dependency Source Maps(使用依赖源码映射)
Assuming you are using a package that uses inline source maps in its distribution, you can use source-map-loader to make webpack aware of them. Without setting it up against the package, you get minified debug output. Often you can skip this step as it’s a special case. 假设你在使用一个包，它在使用内联源码映射，你可以使用 source-map-loader 来让 Webpack 注意到他们。如果没有对这个包进行设置，你会获得较少的输出。由于这是一个特例，通常你可以路过此步。
Source Maps for Styling(样式的源码映射)
If you want to enable source maps for styling files, you can achieve this by enabling the sourceMap option. The same idea works with style loaders such as css-loader, sass-loader, and less-loader. 如果想在样式文件中使用源码映射，可以通过启用 sourceMap 选项来实现。相同的想法对样式加载器如 css-loader, sass-loader 和 less-loader 也适用。
The css-loader is known to have issues when you are using relative paths in imports. To overcome this problem, you should set output.publicPath to resolve the server url. 在 imports 中使用相对路径会导致 css-loader 的已知问题发生. 要解决这个问题，你应该设置 output.publicPath 来处理服务器路径。
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Conclusion（总结）
Source maps can be convenient during development. They provide better means to debug applications as you can still examine the original code over a generated one. They can be valuable even for production usage and allow you to debug issues while serving a client-friendly version of your application. 源码映射给开发带来了便利。他们提供了调试应用程序更好的方法，因为你仍然可以通过生成的代码检查原始代码。他们在生产环境中使用也是很有意义并且让你可以在对用户友好的版本中调试问题。
To recap:(概述)
Source maps can be helpful both during development and production. They provide more accurate information about what’s going on and make it faster to debug possible problems.
源码映射对于开发环境和生产环境都非常有帮助。
Webpack supports a large variety of source map variants. They can be split into inline and separate source maps based on where they are generated. Inline source maps are handy during development due to their speed. Separate source maps work for production as then loading them becomes optional.
Webpack 很多种源码映射变体。他们可以根据生成位置的不同被分成内联式的和分离式的源码映射。因为内联源码映射速度快非常适合开发阶段。由于源码映射加载是可选的, 分离源码映射适合于生产环境.
devtool: “source-map” is the highest quality option making it valuable for production.
devtool: “source-map” 是最高质量的选项，因此非常适合生产环境.
cheap-module-eval-source-map is a good starting point for development.
cheap-module-eval-source-map 是开发的一个好起点。
If you want to get only stack traces during production, use devtool: “hidden-source-map”. You can capture the output and send it to a third party service for you to examine. This way you can capture errors and fix them.
如果只想在生产环境中获得堆栈跟踪，使用 devtool: “hidden-source-map。您可以捕获输出并将其发送到第三方服务以供检查。这样你 能捕获错误并修复他们。
SourceMapDevToolPlugin and EvalSourceMapDevToolPlugin provide more control over the result than the devtool shortcut.
SourceMapDevToolPlugin 和 EvalSourceMapDevToolPlugin 比 devtool 对结果提供更多的控制。
source-map-loader can come in handy if your dependencies provide source maps.
如果依赖包提供源码映射，那么用 source-map-loader 就会很方便。
Enabling source maps for styling requires additional effort. You have to enable sourceMap option per styling related loader you are using.
为式样启用源码映射需要进行额外的配置. 需要为每个加载器启用 sourceMap 选项。
In the next chapter, you’ll learn to split bundles and separate the current one into application and vendor bundles. 在下一节中，你将学到分离 bundle 并将现在的 Bundle 分离到应用 Bundle 和供应商 Bundle 中。
