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| 1 | +Async Code Splitting |
| 2 | +==================== |
| 3 | + |
| 4 | +When you require/import a JavaScript or CSS module, Webpack compiles that code into |
| 5 | +the final JavaScript or CSS file. Usually, that's exactly what you want. But what |
| 6 | +if you only need to use a piece of code under certain conditions? For example, |
| 7 | +what if you want to use `video.js`_ to play a video, but only once a user has |
| 8 | +clicked a link: |
| 9 | + |
| 10 | +.. code-block:: javascript |
| 11 | +
|
| 12 | + // assets/js/app.js |
| 13 | +
|
| 14 | + import $ from 'jquery'; |
| 15 | + // a fictional "large" module (e.g. it imports video.js internally) |
| 16 | + import VideoPlayer from './components/VideoPlayer'; |
| 17 | +
|
| 18 | + $('.js-open-video').on('click', function() { |
| 19 | + // use the larger VideoPlayer module |
| 20 | + const player = new VideoPlayer('some-element'); |
| 21 | + }); |
| 22 | +
|
| 23 | +In this example, the VidePlayer module and everything it imports will be packaged |
| 24 | +into the final, built JavaScript file, even though it may not be very common for |
| 25 | +someone to actually need it. A better solution is to use `dynamic imports`_: load |
| 26 | +the code via AJAX when it's needed: |
| 27 | + |
| 28 | +.. code-block:: javascript |
| 29 | +
|
| 30 | + // assets/js/app.js |
| 31 | +
|
| 32 | + import $ from 'jquery'; |
| 33 | +
|
| 34 | + $('.js-open-video').on('click', function() { |
| 35 | + // you could start a loading animation here |
| 36 | + |
| 37 | + // use import() as a function - it returns a Promise |
| 38 | + import('./components/VideoPlayer').then(({ default: VideoPlayer }) => { |
| 39 | + // you could stop a loading animation here |
| 40 | +
|
| 41 | + // use the larger VideoPlayer module |
| 42 | + const player = new VideoPlayer('some-element'); |
| 43 | +
|
| 44 | + }).catch(error => 'An error occurred while loading the component'); |
| 45 | + }); |
| 46 | +
|
| 47 | +By using ``import()`` like a function, the module will be downloaded async and |
| 48 | +the ``.then()`` callback will be executed when it's finished. The ``VideoPlayer`` |
| 49 | +argument to the callback will be the loaded module. In other words, it works like |
| 50 | +normal AJAX calls! Behind the scenes, Webpack will package the ``VideoPlayer`` module |
| 51 | +into a separate file (e.g. ``0.js``) so it can be downloaded. All the details are |
| 52 | +handled for you. |
| 53 | + |
| 54 | +The ``{ default: VideoPlayer }`` part may look strange. When using the async |
| 55 | +import, your ``.then()`` callback is passed an object, where the *actual* module |
| 56 | +is on a ``.default`` key. There are reasons why this is done, but it does look |
| 57 | +quirky. The ``{ default: VideoPlayer }`` code makes sure that the ``VideoPlayer`` |
| 58 | +module we want is read from this ``.default`` property. |
| 59 | + |
| 60 | +For more details and configuration options, see `dynamic imports`_ on Webpack's |
| 61 | +documentation. |
| 62 | + |
| 63 | +.. _`video.js`: https://videojs.com/ |
| 64 | +.. _`dynamic imports`: https://webpack.js.org/guides/code-splitting/#dynamic-imports |
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