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.listen();
</script>

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

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

ES Module import:

import { listen, prefetch } from "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.listen() once a navigation to a new route has completed
Call quicklink.listen() against a specific DOM element / component
Call quicklink.prefetch() with a custom set of URLs to prefetch

API

quicklink.listen(options)

Returns: Function

A "reset" function is returned, which will empty the active IntersectionObserver and the cache of URLs that have already been prefetched. This can be used between page navigations and/or when significant DOM changes have occurred.

options.el

Type: HTMLElement
Default: document.body

The DOM element to observe for in-viewport links to prefetch.

options.limit

Type: Number
Default: Infinity

The total requests that can be prefetched while observing the options.el container.

options.throttle

Type: Number
Default: Infinity

The concurrency limit for simultaneous requests while observing the options.el container.

options.timeout

Type: Number
Default: 2000

The requestIdleCallback timeout, in milliseconds.

Note: The browser must be idle for the configured duration before prefetching.

options.timeoutFn

Type: Function
Default: requestIdleCallback

A function used for specifying a timeout delay.
This can be swapped out for a custom function like networkIdleCallback (see demos).

By default, this uses requestIdleCallback or the embedded polyfill.

options.priority

Type: Boolean
Default: false

Whether or not the URLs within the options.el container should be treated as high priority.

When true, quicklink will attempt to use the fetch() API if supported (rather than link[rel=prefetch]).

options.origins

Type: Array<String>
Default: [location.hostname]

A static array of URL hostnames that are allowed to be prefetched.
Defaults to the same domain origin, which prevents any cross-origin requests.

Important: An empty array ([]) allows all origins to be prefetched.

options.ignores

Type: RegExp or Function or Array
Default: []

Determine if a URL should be prefetched.

When a RegExp tests positive, a Function returns true, or an Array contains the string, then the URL is not prefetched.

Note: An Array may contain String, RegExp, or Function values.

Important: This logic is executed after origin matching!

options.onError

Type: Function
Default: None

An optional error handler that will receive any errors from prefetched requests.
By default, these errors are silently ignored.

quicklink.prefetch(urls, isPriority)

Returns: Promise

The urls provided are always passed through Promise.all, which means the result will always resolve to an Array.

Important: You much catch you own request error(s).

urls

Type: String or Array<String>
Required: true

One or many URLs to be prefetched.

Note: Each url value is resolved from the current location.

isPriority

Type: Boolean
Default: false

Whether or not the URL(s) should be treated as "high priority" targets.
By default, calls to prefetch() are low priority.

Note: This behaves identically to listen()'s priority option.

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/v3/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.listen({
  timeout: 4000
});

Set the DOM element to observe for in-viewport links

Defaults to document otherwise.

quicklink.listen({
  el: document.getElementById('carousel')
});

Programmatically `prefetch()` URLs

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

// Single URL
quicklink.prefetch('2.html');

// Multiple URLs
quicklink.prefetch(['2.html', '3.html', '4.js']);

// Multiple URLs, with high priority
// Note: Can also be use with single URL!
quicklink.prefetch(['2.html', '3.html', '4.js'], true);

Set the request priority for prefetches while scrolling

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

Note: This runs prefetch(..., true) with URLs found within the options.el container.

quicklink.listen({ 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.listen({
  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.listen({
  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.listen({
  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.listen({
  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, Safari ≥ 12.1, Firefox, Edge, Opera, Android Browser, Samsung Internet.
With Intersection Observer polyfill ~6KB gzipped/minified: Safari ≤ 12.0, 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

A prefetch method can be individually imported for use in other projects.
This method includes the logic to respect Data Saver and 2G connections. It also issues requests thru fetch(), XHRs, or link[rel=prefetch] depending on (a) the isPriority value and (b) the current browser's support.

After installing quicklink as a dependency, you can use it as follows:

<script type="module">
  import { prefetch } from 'quicklink';
  prefetch(['1.html', '2.html']).catch(err => {
    // Handle own errors
  });
</script>

Demo

Glitch demos

Research

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.

Additional notes

Session Stitching

Cross-origin prefetching (e.g a.com/foo.html prefetches b.com/bar.html) has a number of limitations. One such limitation is with session-stitching. b.com may expect a.com's navigation requests to include session information (e.g a temporary ID - e.g b.com/bar.html?hash=<>&timestamp=<>), where this information is used to customize the experience or log information to analytics. If session-stitching requires a timestamp in the URL, what is prefetched and stored in the HTTP cache may not be the same as the one the user ultimately navigates to. This introduces a challenge as it can result in double prefetches.

To workaround this problem, you can consider passing along session information via the ping attribute (separately) so the origin can stitch a session together asynchronously.

Ad-related considerations

Sites that rely on ads as a source of monetization should not prefetch ad-links, to avoid unintentionally counting clicks against those ad placements, which can lead to inflated Ad CTR (click-through-rate).

Ads appear on sites mostly in two ways:

Inside iframes: By default, most ad-servers render ads within iframes. In these cases, those ad-links won't be prefetched by Quicklink, unless a developer explicitly passes in the URL of an ads iframe. The reason is that the library look-up for in-viewport elements is restricted to those of the top-level origin.
Outside iframes:: In cases when the site shows same-origin ads, displayed in the top-level document (e.g. by hosting the ads themselves and by displaying the ads in the page directly), the developer needs to explicitly tell Quicklink to avoid prefetching these links. This can be achieved by passing the URL or subpath of the ad-link, or the element containing it to the custom ignore patterns list.

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.
WordPress users can now get quicklink as a WordPress Plugin from the plugin repository.
Drupal users can install the Quicklink Drupal module.
Want less aggressive prefetching? instant.page prefetches on mouseover and touchstart, right before a click.

License

Licensed under the Apache-2.0 license.