fetch-inject

fetchInject

Implements Fetch Injection to dynamically inline assets into the DOM using the Fetch API, with support for promise chains.

Purpose

Use to inject remote assets into the DOM, then do something with them. Currently supports inlining of script and style elements.

Usage

1. Call fetchInject with an array of URLs.
2. Optionally, handle the returned Promise.

Use Cases

Loading Utility Scripts

Problem: You want to prototype some code using the browser as a REPL.

Solution: Skip the emulators and use the real deal:

fetchInject([
  'https://cdn.jsdelivr.net/lodash/4.17.4/lodash.min.js'
]).then(() => {
  console.log(`Successfully loaded Lodash ${_.VERSION}`)
})

Loading CSS Asynchronously

Problem: PageSpeed Insights dings you for loading unnecessary styles on initial render.

Solution: Inline your critical path CSS and load non-critical styles asynchronously:

<style>
  *{box-sizing:border-box;text-rendering:geometricPrecision}
</style>
<script>
  fetchInject([
    '/css/non-critical.css',
    'https://cdn.jsdelivr.net/fontawesome/4.7.0/css/font-awesome.min.css'
  ])
</script>

Preventing Script Blocking

Problem: Remote assets can lead to jank or, worse yet, SPOF if not handled correctly.

Solution: Asynchronously load remote scripts without blocking:

<script>fetchInject(['https://use.typekit.net/spoful8r.js'])</script>

Loading Scripts Lazily

Problem: You want to load a script in response to a event without optimistically preloading the asset.

Solution: Create an event listener, respond to the event and then destroy the listener.

const el = document.querySelector('details summary')
el.onclick = (evt) => {
  fetchInject([
    'https://cdn.jsdelivr.net/smooth-scroll/10.2.1/smooth-scroll.min.js'
  ])
  el.onclick = null  
}

Responding to Asynchronous Scripts

Problem: You need to perform a synchronous operation immediately after an asynchronous script is loaded.

Solution: You could create a script element and use the async and onload attributes. Or you could...

fetchInject([
  'https://cdn.jsdelivr.net/momentjs/2.17.1/moment.min.js'
]).then(() => {
  console.log(`Finish in less than ${moment().endOf('year').fromNow()}`)
})

Combining Resource Types

Problem: You need to asynchronously download multiple related assets of different types.

Solution: Specify multiple URLs of different types when calling:

fetchInject([
  'https://cdn.jsdelivr.net/normalize/5.0.0/normalize.css',
  '//cdnjs.cloudflare.com/ajax/libs/tether/1.4.0/js/tether.min.js',
])

Ordering Dependent Scripts

Problem: You have several scripts that depend on one another and you want to load them all asynchronously without causing race conditions.

Solution: Call multiple times, forming a promise chain:

fetchInject([
  'https://cdn.jsdelivr.net/jquery/3.1.1/jquery.slim.min.js'
]).then(() => {
  fetchInject([
    'https://npmcdn.com/tether@1.2.4/dist/js/tether.min.js'
  ]).then(() => {
    fetchInject([
      'https://npmcdn.com/bootstrap@4.0.0-alpha.5/dist/js/bootstrap.min.js'
    ])
  })
})

Loading and Handling Composite Libraries

Problem: You want to use library made up of a number of different resources, and then instantiate it upon completion.

Solution: This is precisely why fetchInject was created:

const container = document.querySelectorAll('.pswp')[0]
const items = JSON.parse({{ .Params.gallery.main | jsonify }})
fetchInject([
  '/css/photoswipe.css',
  '/css/default-skin/default-skin.css',
  '/js/photoswipe.min.js',
  '/js/photoswipe-ui-default.min.js'
]).then(() => {
  const gallery = new PhotoSwipe(container, PhotoSwipeUI_Default, items)
  gallery.init()
})

Known Limitations

• Requires you have at least one of script or style in the head of your document already, depending on which you're injecting.
• Does not perform any caching.
• Does not support Isomorphic rendering.

Supported Browsers

All browsers with native support the Fetch API and ES2015.

Development

1. Clone the repo with git clone https://github.com/jhabdas/fetch-inject.git.
2. Install dependencies with npm i (brew install node first on macOS).
3. Modify files in src directory and run npm test to lint the package.
4. Create a distributable package using npm run build.

Note: Build variants possible for various module types via the format setting in rollup.config.js. I intend to switch to ES6 modules as soon as reasonable browser support is realized.

Related Libraries

• window.fetch polyfill
• loadCSS

Copyright (c) 2017 Josh Habdas