quicklink

Faster subsequent page-loads by prefetching in-viewport links during idle time

How it works

Quicklink attempts to make navigations to subsequent pages load faster. It:

Detects links within the viewport (using Intersection Observer)
Waits until the browser is idle (using requestIdleCallback)
Checks if the user isn't on a slow connection (using navigator.connection.effectiveType) or has data-saver enabled (using navigator.connection.saveData)
Prefetches URLs to the links (using <link rel=prefetch> or XHR). Provides some control over the request priority (can switch to fetch() if supported).

Why

This project aims to be a drop-in solution for sites to prefetch links based on what is in the user's viewport. It also aims to be small (< 1KB minified/gzipped).

Installation

For use with node and npm:

npm install --save quicklink

You can also grab quicklink from unpkg.com/quicklink.

Usage

Once initialized, quicklink will automatically prefetch URLs for links that are in-viewport during idle time.

Quickstart:

<!-- Include quicklink from dist -->
<script src="dist/quicklink.umd.js"></script>
<!-- Initialize (you can do this whenever you want) -->
<script>
quicklink();
</script>

For example, you can initialize after the load event fires:

<script>
window.addEventListener('load', () =>{
   quicklink();
});
</script>

ES Module import:

import quicklink from "quicklink/dist/quicklink.mjs";
quicklink();

The above options are best for multi-page sites. Single-page apps have a few options available for using quicklink with a router:

Call quicklink() once a navigation to a new route has completed
Call quicklink() against a specific DOM element / component
Call quicklink({urls:[...]}) with a custom set of URLs to prefetch

API

quicklink accepts an optional options object with the following parameters:

el: DOM element to observe for in-viewport links to prefetch
urls: Static array of URLs to prefetch (instead of observing document or a DOM element links in the viewport)
timeout: Integer for the requestIdleCallback timeout. A time in milliseconds by which the browser must execute prefetching. Defaults to 2 seconds.
timeoutFn: Function for specifying a timeout. Defaults to requestIdleCallback. Can also be swapped out for a custom function like networkIdleCallback (see demos)
priority: Boolean specifying preferred priority for fetches. Defaults to false. true will attempt to use the fetch() API where supported (rather than rel=prefetch)
origins: Static array of URL hostname strings that are allowed to be prefetched. Defaults to the same domain origin, which prevents any cross-origin requests.
ignores: A RegExp, Function, or Array that further determines if a URL should be prefetched. These execute after origin matching.

TODO:

Explore detecting file-extension of resources and using rel=preload for high priority fetches
Explore using Priority Hints for importance hinting

Polyfills

quicklink:

Includes a very small fallback for requestIdleCallback
Requires IntersectionObserver to be supported (see CanIUse). We recommend conditionally polyfilling this feature with a service like Polyfill.io:

<script src="https://polyfill.io/v2/polyfill.min.js?features=IntersectionObserver"></script>

Alternatively, see the Intersection Observer polyfill.

Recipes

Set a custom timeout for prefetching resources

Defaults to 2 seconds (via requestIdleCallback). Here we override it to 4 seconds:

quicklink({
  timeout: 4000
});

Set the DOM element to observe for in-viewport links

Defaults to document otherwise.

const elem = document.getElementById('carousel');
quicklink({
  el: elem
});

Set a custom array of URLs to be prefetched

If you would prefer to provide a static list of URLs to be prefetched, instead of detecting those in-viewport, customizing URLs is supported.

quicklink({
   urls: ['2.html','3.html', '4.js']
});

Set the request priority for prefetches

Defaults to low-priority (rel=prefetch or XHR). For high-priority (priority: true), attempts to use fetch() or falls back to XHR.

quicklink({ priority: true });

Specify a custom list of allowed origins

Provide a list of hostnames that should be prefetch-able. Only the same origin is allowed by default.

Important: You must also include your own hostname!

quicklink({
  origins: [
    // add mine
    'my-website.com',
    'api.my-website.com',
    // add third-parties
    'other-website.com',
    'example.com',
    // ...
  ]
});

Allow all origins

Enables all cross-origin requests to be made.

Note: You may run into CORB and CORS issues!

quicklink({
  origins: true,
  // or
  origins: []
});

Custom Ignore Patterns

These filters run after the origins matching has run. Ignores can be useful for avoiding large file downloads or for responding to DOM attributes dynamically.

// Same-origin restraint is enabled by default.
//
// This example will ignore all requests to:
//  - all "/api/*" pathnames
//  - all ".zip" extensions
//  - all <a> tags with "noprefetch" attribute
//
quicklink({
  ignores: [
    /\/api\/?/,
    uri => uri.includes('.zip'),
    (uri, elem) => elem.hasAttribute('noprefetch')
  ]
});

You may also wish to ignore prefetches to URLs which contain a URL fragment (e.g. index.html#top). This can be useful if you (1) are using anchors to headings in a page or (2) have URL fragments setup for a single-page application, and which to avoid firing prefetches for similar URLs.

Using ignores this can be achieved as follows:

quicklink({
    ignores: [
        uri => uri.includes('#')
        // or RegExp: /#(.+)/
        // or element matching: (uri, elem) => !!elem.hash
    ]
});

Browser support

The prefetching provided by quicklink can be viewed as a progressive enhancement. Cross-browser support is as follows:

Without polyfills: Chrome, Firefox, Edge, Opera, Android Browser, Samsung Internet.
With Intersection Observer polyfill ~6KB gzipped/minified: Safari, IE11
With the above and a Set() and Array.from polyfill: IE9 and IE10. Core.js provides both Set() and Array.from() shims. Projects like es6-shim are an alternative you can consider.

Certain features have layered support:

The Network Information API, which is used to check if the user has a slow effective connection type (via navigator.connection.effectiveType) is only available in Chrome 61+ and Opera 57+
If opting for {priority: true} and the Fetch API isn't available, XHR will be used instead.

Using the prefetcher directly

quicklink includes a prefetcher that can be individually imported for use in other projects. After installing quicklink as a dependency, you can use it as follows:

<script type="module">
import prefetch from '../src/prefetch.mjs';

const urls = ['1.html', '2.html'];
const promises = urls.map(url => prefetch(url));
Promise.all(promises);
</script>

Demo

Here's a WebPageTest run for our demo improving page-load performance by up to 4 seconds via quicklink's prefetching. A video comparison of the before/after prefetching is on YouTube.

For demo purposes, we deployed a version of the Google Blog on Firebase hosting. We then deployed another version of it, adding quicklink to the homepage and benchmarked navigating from the homepage to an article that was automatically prefetched. The prefetched version loaded faster.

Please note: this is by no means an exhaustive benchmark of the pros and cons of in-viewport link prefetching. Just a demo of the potential improvements the approach can offer. Your own mileage may heavily vary.

Related projects

Using Gatsby? You already get most of this for free baked in. It uses Intersection Observer to prefetch all of the links that are in view and provided heavy inspiration for this project.
Want a more data-driven approach? See Guess.js. It uses analytics and machine-learning to prefetch resources based on how users navigate your site. It also has plugins for Webpack and Gatsby.

License

Licensed under the Apache-2.0 license.

lirunkai/quicklink