前端-工程打包時，為js, css等檔案的檔名做hash處理，以處理客戶端瀏覽器快取舊程式碼的情況，當有新版本的程式碼時客戶端可能會有這樣的不確定性的問題

對於angular的工程可以這樣打包：ng build -prod --output-hashing=all

But those hashed bundles are great, because it solves a common issue with browser and proxy servers caching old versions of your scripts. Don't underestimate this as it can generate all kinds of strange behavior for users.

That means we need a solution which works with dynamically changing file names, just having URLs in the template wouldn't do it!

http://michaco.net/blog/Angular4GettingHashedWebpackBundlesWorkingInASPNETCoreMVC

// Adding Hashes to Filenames

https://survivejs.com/webpack/optimizing/adding-hashes-to-filenames/

Unicon - Wrangle SVGs from your favorite design tool - Interview with Travis Arnold

Even though the generated build works the file names it uses is problematic. It doesn't allow to leverage client level cache efficiently as there's no way tell whether or not a file has changed. Cache invalidation can be achieved by including a hash to the filenames.

Placeholders

Webpack provides placeholders for this purpose. These strings are used to attach specific information to webpack output. The most valuable ones are:

• [id] - Returns the chunk id.
• [path] - Returns the file path.
• [name] - Returns the file name.
• [ext] - Returns the extension. [ext] works for most available fields. MiniCssExtractPlugin is a notable exception to this rule.
• [hash] - Returns the build hash. If any portion of the build changes, this changes as well.
• [chunkhash] - Returns an entry chunk-specific hash. Each entrydefined in the configuration receives a hash of its own. If any portion of the entry changes, the hash will change as well. [chunkhash] is more granular than [hash] by definition.
• [contenthash] - Returns a hash generated based on content.

It's preferable to use particularly hash and chunkhash only for production purposes as hashing doesn't do much good during development.

It's possible to slice  hash and  chunkhash using specific syntax:  [chunkhash:4]. Instead of a hash like  8c4cbfdb91ff93f3f3c5 this would yield  8c4c.

There are more options available, and you can even modify the hashing and digest type as discussed at  loader-utils documentation.

Example Placeholders

Assume you have the following configuration:

{
  output: {
    path: PATHS.build,
    filename: "[name].[chunkhash].js",
  },
},

Webpack would generate filenames like these based on it:

main.d587bbd6e38337f5accd.js
vendor.dc746a5db4ed650296e1.js

If the file contents related to a chunk are different, the hash changes as well, thus the cache gets invalidated. More accurately, the browser sends a new request for the new file. If only main bundle gets updated, only that file needs to be requested again.

The same result can be achieved by generating static filenames and invalidating the cache through a querystring (i.e., main.js?d587bbd6e38337f5accd). The part behind the question mark invalidates the cache. According to Steve Souders, attaching the hash to the filename is the most performant option.

Setting Up Hashing

The build needs tweaking to generate proper hashes. Images and fonts should receive hash while chunks should use chunkhash in their names to invalidate them correctly:

webpack.config.js

const productionConfig = merge([

{ output: { chunkFilename: "[name].[chunkhash:4].js", filename: "[name].[chunkhash:4].js", }, },

... parts.loadImages({ options: { limit: 15000,

name: "[name].[ext]",

name: "[name].[hash:4].[ext]",

}, }), ... ]);

[hash] is defined differently for  file-loader than for the rest of webpack. It's calculated based on file  content. See  file-loader documentation for further information.

If you used chunkhash for the extracted CSS as well, this would lead to problems as the code points to the CSS through JavaScript bringing it to the same entry. That means if the application code or CSS changed, it would invalidate both.

Therefore, instead of chunkhash, you can use contenthash that is generated based on the extracted content:

webpack.parts.js

exports.extractCSS = ({ include, exclude, use }) => {
  // Output extracted CSS to a file
  const plugin = new MiniCssExtractPlugin({

filename: "[name].css",

filename: "[name].[contenthash:4].css",

}); ... };

The hashes have been sliced to make the output fit better in the book. In practice, you can skip slicing them.

If you generate a build now (npm run build), you should see something:

Hash: fb67c5fd35454da1d6ff
Version: webpack 4.1.1
Time: 3034ms
Built at: 3/16/2018 6:18:07 PM
                   Asset       Size  Chunks             Chunk Names
               0.0847.js  161 bytes       0  [emitted]
    vendors~main.d2f1.js   96.8 KiB       1  [emitted]  vendors~main
            main.745c.js   2.25 KiB       2  [emitted]  main
           main.5524.css    1.2 KiB       2  [emitted]  main
   vendors~main.3dd5.css   1.32 KiB       1  [emitted]  vendors~main
           0.0847.js.map  203 bytes       0  [emitted]
vendors~main.d2f1.js.map    235 KiB       1  [emitted]  vendors~main
        main.745c.js.map   11.4 KiB       2  [emitted]  main
              index.html  349 bytes          [emitted]
Entrypoint main = vendors~main.d2f1.js ...
...

The files have neat hashes now. To prove that it works for styling, you could try altering src/main.css and see what happens to the hashes when you rebuild.

There's one problem, though. If you change the application code, it invalidates the vendor file as well! Solving this requires extracting a manifest, but before that, you can improve the way the production build handles module IDs.

Conclusion

Including hashes related to the file contents to their names allows to invalidate them on the client side. If a hash has changed, the client is forced to download the asset again.

To recap:

• Webpack's placeholders allow you to shape filenames and enable you to include hashes to them.
• The most valuable placeholders are [name], [chunkhash], and [ext]. A chunk hash is derived based on the entry in which the asset belongs.
• If you are using MiniCssExtractPlugin, you should use [contenthash]. This way the generated assets get invalidated only if their content changes.

Even though the project generates hashes now, the output isn't flawless. The problem is that if the application changes, it invalidates the vendor bundle as well. The next chapter digs deeper into the topic and shows you how to extract a manifest to resolve the issue.