To keep JavaScript sources small, it can sometimes make sense to split your webpack bundles. For example, if your website uses some large JavaScript library – say TinyMCE – which is only required on some selected pages, it makes sense to only load that library when necessary.

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In modern webpack this is easily doable by using the asynchronous import function.

Say we have an unpoly compiler that sets up TinyMCE like this (code is somewhat simplified):

// TinyMCE as part of the main bundle!

import tinymce from 'tinymce/tinymce'

// UI
import 'tinymce/themes/silver'
import 'tinymce/skins/ui/oxide/skin.min.css'

// Plugin
import 'tinymce/plugins/autoresize'

up.compiler('[tinymce]', (element) => {
  let editor = undefined

  tinymce.init({
    target: element,
    plugins: ['autoresize'],
  }).then((editors) => {
    editor = editors[0]
  })

  return () => {
    if (editor) {
      editor.destroy()
    }
  }
})

Note that import is used as a keyword. This causes webpack to bundle TinyMCE and its dependencies into the main bundle. However, we can instead use the import function to make webpack create a separate bundle:

import('tinymce/tinymce').then(({ default: tinymce }) => {
  // this gets called after tinymce has been loaded via AJAX
})

The import function allows webpack to automatically put the imported JavaScript into a separate bundle (it uses some heuristic to decide whether to do it), automatically load that bundle via AJAX, and resolve a promise as soon as the JavaScript has been loaded.

The compiler above can thus be rewritten like this:

// TinyMCE is a separate bundle!

import 'tinymce/skins/ui/oxide/skin.min.css' // CSS is still put into the main bundle

up.compiler('[tinymce]', (element) => {
  let editor = undefined

  loadTinyMce()
  .then(tinyMce => {
    return tinymce.init({
      target: element,
      plugins: ['autoresize'],
    })
  })
  .then((editors) => {
    editor = editors[0]
  })

  function loadTinyMce() {
    return Promise.all([
      import(/* webpackChunkName: "tinymce" */ 'tinymce/tinymce'),
      // UI styles
      import(/* webpackChunkName: "tinymce" */ 'tinymce/themes/silver'),
      // Plugins
      import(/* webpackChunkName: "tinymce" */ 'tinymce/plugins/autoresize'),
    ])
    .then((imports) => {
      return imports[0].default // tinymce, i.e. the default export of the tinymce/tinymce package
    })
  }

  return () => {
    if (editor) {
      editor.destroy()
    }
  }
})

The magic webpackChunkName comments force webpack to put all those files into a bundle of the given name, overriding webpacks own heuristic.