A webpack plugin for injecting <link rel='preload|prefecth'> into HtmlWebpackPlugin pages, with async chunk support
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README.md

preload-webpack-plugin

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preloads-plugin-compressor

A Webpack plugin for automatically wiring up asynchronous (and other types) of JavaScript chunks using <link rel='preload'>. This helps with lazy-loading.

Note: This is an extension plugin for html-webpack-plugin - a plugin that simplifies the creation of HTML files to serve your webpack bundles.

This plugin is a stop-gap until we add support for asynchronous chunk wiring to script-ext-html-webpack-plugin.

Introduction

Preload is a web standard aimed at improving performance and granular loading of resources. It is a declarative fetch that can tell a browser to start fetching a source because a developer knows the resource will be needed soon. Preload: What is it good for? is a recommended read if you haven't used the feature before.

In simple web apps, it's straight-forward to specify static paths to scripts you would like to preload - especially if their names or locations are unlikely to change. In more complex apps, JavaScript can be split into "chunks" (that represent routes or components) at with dynamic names. These names can include hashes, numbers and other properties that can change with each build.

For example, chunk.31132ae6680e598f8879.js.

To make it easier to wire up async chunks for lazy-loading, this plugin offers a drop-in way to wire them up using <link rel='preload'>.

Pre-requisites

This module requires Webpack 2.2.0 and above. It also requires that you're using html-webpack-plugin in your Webpack project.

Installation

First, install the package as a dependency in your package.json:

$ npm install --save-dev preload-webpack-plugin

Alternatively, using yarn:

yarn add -D preload-webpack-plugin

Usage

Next, in your Webpack config, require() the preload plugin as follows:

const PreloadWebpackPlugin = require('preload-webpack-plugin');

and finally, configure the plugin in your Webpack plugins array after HtmlWebpackPlugin:

plugins: [
  new HtmlWebpackPlugin(),
  new PreloadWebpackPlugin()
]

When preloading files, the plugin will use different as attribute depends on the type of each file. For each file ends with .css, the plugin will preload it with as=style, for each file ends with .woff2, the plugin will preload it with as=font, while for all other files, as=script will be used.

If you do not prefer to determine as attribute depends on suffix of filename, you can also explicitly name it using as:

plugins: [
  new HtmlWebpackPlugin(),
  new PreloadWebpackPlugin({
    rel: 'preload',
    as: 'script'
  })
]

In case you need more fine-grained control of the as atribute, you could also provide a function here. When using it, entry name will be provided as the parameter, and function itself should return a string for as attribute:

plugins: [
  new HtmlWebpackPlugin(),
  new PreloadWebpackPlugin({
    rel: 'preload',
    as(entry) {
      if (/\.css$/.test(entry)) return 'style';
      if (/\.woff$/.test(entry)) return 'font';
      if (/\.png$/.test(entry)) return 'image';
      return 'script';
    }
  })
]

Notice that if as=font is used in preload, crossorigin will be added, otherwise the font resource might be double fetched. Explains can be found in this article.

By default, the plugin will assume async script chunks will be preloaded. This is the equivalent of:

plugins: [
  new HtmlWebpackPlugin(),
  new PreloadWebpackPlugin({
    rel: 'preload',
    include: 'asyncChunks'
  })
]

For a project generating two async scripts with dynamically generated names, such as chunk.31132ae6680e598f8879.js and chunk.d15e7fdfc91b34bb78c4.js, the following preloads will be injected into the document <head>:

<link rel="preload" as="script" href="chunk.31132ae6680e598f8879.js">
<link rel="preload" as="script" href="chunk.d15e7fdfc91b34bb78c4.js">

You can also configure the plugin to preload all chunks (vendor, async, normal chunks) using include: 'allChunks', or only preload initial chunks with include: 'initial':

plugins: [
  new HtmlWebpackPlugin(),
  new PreloadWebpackPlugin({
    rel: 'preload',
    include: 'allChunks' // or 'initial'
  })
]

In case you work with named chunks, you can explicitly specify which ones to include by passing an array:

plugins: [
  new HtmlWebpackPlugin(),
  new PreloadWebpackPlugin({
    rel: 'preload',
    include: ['home']
  })
]

will inject just this:

<link rel="preload" as="script" href="home.31132ae6680e598f8879.js">

It is very common in Webpack to use loaders such as file-loader to generate assets for specific types, such as fonts or images. If you wish to preload these files as well, you can use include with value allAssets:

plugins: [
  new HtmlWebpackPlugin(),
  new PreloadWebpackPlugin({
    rel: 'preload',
    include: 'allAssets',
  })
]

One thing worth noticing: file-loader provides an option to specify publicPath just for assets. However, that information seems to be lost when this plugin is doing its job. Thus, this plugin will use publicPath defined in output config. It could be an issue, if publicPath in file-loader and publicPath in webpack config have different values.

Usually you don't want to preload all of them but only keep the necessary resources, you can use fileBlacklist or fileWhitelist shown below to filter.

Filtering chunks

There may be chunks that you don't want to have preloaded (sourcemaps, for example). Before preloading each chunk, this plugin checks that the file does not match any regex in the fileBlacklist option. The default value of this blacklist is [/\.map/], meaning no sourcemaps will be preloaded. You can easily override this:

new PreloadWebpackPlugin({
  fileBlacklist: [/\.whatever/]
})

Passing your own array will override the default, so if you want to continue filtering sourcemaps along with your own custom settings, you'll need to include the regex for sourcemaps:

new PreloadWebpackPlugin({
  fileBlacklist: [/\.map/, /\.whatever/]
})

If you use include="allAssets", you might find excluding all unnecessary files one by one a bit annoying. In this case, you can use fileWhitelist to only include the files you want:

new PreloadWebpackPlugin({
  fileWhitelist: [/\.files/, /\.to/, /\.include/],
})

notice that if fileWhitelist is not provided, it will not filter any file out.

Also, you could use fileWhitelist and fileBlacklist together:

new PreloadWebpackPlugin({
  fileWhitelist: [/\.files/, /\.to/, /\.include/],
  fileBlacklist: [/\.files/, /\.to/, /\.exclude/],
})

In example above, only files with name matches /\.include/ will be included.

Filtering HTML

In some case, you may don't want to preload resource on some file. But using fileBlacklist is weird, because you may want to inlcude this chunk on another file. So you can use excludeHtmlNames to tell preload plugin to ignore this file.

If you have multiple html like index.html and example.html, you can exclude index.html like this.

plugins: [
  new HtmlWebpackPlugin({
    filename: 'index.html',
    template: 'src/index.html',
    chunks: ['main']
  }),
  new HtmlWebpackPlugin({
    filename: 'example.html',
    template: 'src/example.html',
    chunks: ['exampleEntry']
  }),
  // I want this to affect only index.html
  new PreloadWebpackPlugin({
    excludeHtmlNames: ['index.html'],
  })

Resource Hints

Should you wish to use Resource Hints (such as prefetch) instead of preload, this plugin also supports wiring those up.

Prefetch:

plugins: [
  new HtmlWebpackPlugin(),
  new PreloadWebpackPlugin({
    rel: 'prefetch'
  })
]

For the async chunks mentioned earlier, the plugin would update your HTML to the following:

<link rel="prefetch" href="chunk.31132ae6680e598f8879.js">
<link rel="prefetch" href="chunk.d15e7fdfc91b34bb78c4.js">

Demo

A demo application implementing the PRPL pattern with React that uses this plugin can be found in the demo directory.

Support

If you've found an error in this sample, please file an issue: https://github.com/googlechrome/preload-webpack-plugin/issues

Patches are encouraged, and may be submitted by forking this project and submitting a pull request through GitHub.

Contributing workflow

index.js contains the primary source for the plugin, test contains tests and demo contains demo code.

Test the plugin:

$ npm install
$ npm run test

Lint the plugin:

$ npm run lint
$ npm run lint-fix # fix linting issues

The project is written in ES2015, but does not use a build-step. This may change depending on any Node version support requests posted to the issue tracker.

Additional Notes

  • Be careful not to preload resources a user is unlikely to need. This can waste their bandwidth.
  • Use preload for the current session if you think a user is likely to visit the next page. There is no 100% guarantee preloaded items will end up in the HTTP Cache and read locally beyond this session.
  • If optimising for future sessions, use prefetch and preconnect. Prefetched resources are maintained in the HTTP Cache for at least 5 minutes (in Chrome) regardless of the resource's cachability.

Related plugins

License

Copyright 2017 Google, Inc.

Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to you under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.