fetch-inject

Fetch Inject

A library used to dynamically inline content into the DOM using Fetch Injection.

Background

Fetch Injection is performance optimization technique for loading resources into the DOM asynchronously using the Fetch API. Use it to fetch async resources in parallel (even across the network), and inject them into a document programmatically in a desired sequence.

Understand why this library exists by reading the intro article on Hack Cabin.

Don't have time to read? Here's a playground:
https://codepen.io/jhabdas/pen/MpVeOE?editors=0012

Installing

Fetch Inject is available on NPM, Bower and CDN.

• Get it on NPM with npm i fetch-inject,
  
• Bower with bower install fetch-inject; or,
  
• CDN using jsDelivr.

Syntax

Promise<Array<Object>> fetchInject(inputs, promise)

Parameters

inputs

Required. This defines the resources you wish to fetch. It must be an Array containing elements of type USVString or Request.

promise

Optional. A Promise to await before injecting fetched resources.

Return value

A Promise that resolves to an Array of Objects. Each Object contains a list of resolved properties of the Response Body used in the module, e.g.

[{
  blob: { size: 44735, type: "application/javascript" },
  text: "/*!↵ * Bootstrap v4.0.0-alpha.5 ... */"
}, {
  blob: { size: 31000, type: "text/css" },
  text: "/*!↵ * Font Awesome 4.7.0 ..."
}]

Use Cases

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:

fetchInject([
  '/bower_components/jquery/dist/jquery.js',
  '/bower_components/what-input/dist/what-input.js'
])

Loading CSS Asynchronously

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

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

fetchInject([
  '/css/non-critical.css',
  'https://cdn.jsdelivr.net/fontawesome/4.7.0/css/font-awesome.min.css'
])

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(true)}`)
})

Ordering Dependent Scripts

Problem: You have several scripts that depend on one another and you want to load them all asynchronously, in parallel, without causing a race condition.

Solution: Pass fetchInject to itself as a second argument, forming a recursion:

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

Loading and Handling Composite Libraries

Problem: You want to use library composed of several resources and initialize it as soon as possible.

Solution: This is precisely the kind of activity fetchInject works best at:

const container = document.querySelector('.pswp')
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()
})

Managing Asynchronous Dependencies

Problem: You want to load some dependencies which require some dependencies, which require a dependency. You want it all in parallel, and you want it now.

Solution: You could scatter some links into your document head, blocking initial page render. You could bloat your application bundle with scripts the user might not actually need half the time. Or you could...

const tether = ['https://cdn.jsdelivr.net/tether/1.4.0/tether.min.js']
const drop = [
  'https://cdn.jsdelivr.net/drop/1.4.2/js/drop.min.js'
]
const tooltip = [
  'https://cdn.jsdelivr.net/tooltip/1.2.0/tooltip.min.js',
  'https://cdn.jsdelivr.net/tooltip/1.2.0/tooltip-theme-arrows.css'
]
fetchInject(tooltip,
  fetchInject(drop,
    fetchInject(tether)
  )
).then(() => {
  new Tooltip({
    target: document.querySelector('h1'),
    content: "You moused over the first <b>H1</b>!",
    classes: 'tooltip-theme-arrows',
    position: 'bottom center'
  })
})

Supported Browsers

All browsers with support for Fetch and Promises.

Fetch will become available in Safari in the Safari 10.1 release that ships with macOS Sierra 10.12.4 and Safari on iOS 10.3. Jon Davis,  Web Technologies Evangelist

Development

1. Clone the repo with git clone https://github.com/jhabdas/fetch-inject.
2. Install dev dependencies with npm i.
3. Execute npm run for a listing of available commands.

Contributing

Please use Issues for bugs and enhancement requests only. Use npm run commit to create Commitizen-friendly commit messages. If you need support, you know where to go. Thanks!

See Also

• fetch - Polyfill for window.fetch
• promise-polyfill - Polyfill for Promises
• es-module-loader - Polyfill for the ES Module Loader
• isomorphic-fetch - A library for using fetch in Node

License

ISC