* **Prefetches** (using [``](https://www.w3.org/TR/resource-hints/#prefetch) or XHR) or **prerenders** (using [Speculation Rules API](https://github.com/WICG/nav-speculation/blob/main/triggers.md)) URLs to the links. Provides some control over the request priority (can switch to `fetch()` if supported).Type: `Boolean`
Type: `Number`
Type: `HTMLElement`
Type: `Number`
Type: `Number`
Type: `Number`
Type: `Number`
Type: `Function`
A function used for specifying a `timeout` delay.
Type: `Boolean`
Type: `Array`

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.

options.hrefFn

Type: `Function`

Default: None

An optional function to generate the URL to prefetch. It receives an Element as the argument.

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`

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.

quicklink.prerender(urls)

Returns: Promise

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

urls

Type: `String` or `Array`

Required: true

One or many URLs to be prerendered.

Note: As prerendering using Speculative Rules API only supports same-origin at this point, only same-origin urls are accepted. Any non same-origin urls will return a rejected Promise.

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:

0. ``` html
1. <script src="https://polyfill.io/v3/polyfill.min.js?features=IntersectionObserver"></script>
2. ```

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:

0. ``` js
1. quicklink.listen({
2.   timeout: 4000
3. });
4. ```

Set the DOM element to observe for in-viewport links

Defaults to document otherwise.

0. ``` js
1. quicklink.listen({
2.   el: document.getElementById('carousel')
3. });
4. ```

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.

0. ``` js
1. // Single URL
2. quicklink.prefetch('2.html');
3. 
   
4. // Multiple URLs
5. quicklink.prefetch(['2.html', '3.html', '4.js']);
6. 
   
7. // Multiple URLs, with high priority
8. // Note: Can also be use with single URL!
9. quicklink.prefetch(['2.html', '3.html', '4.js'], true);
10. ```

Programmatically prerender() URLs

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

0. ``` js
1. // Single URL
2. quicklink.prerender('2.html');
3. 
   
4. // Multiple URLs
5. quicklink.prerender(['2.html', '3.html', '4.js']);
6. ```

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.

0. ``` js
1. quicklink.listen({ priority: true });
2. ```

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!

0. ``` js
1. quicklink.listen({
2.   origins: [
3.     // add mine
4.     'my-website.com',
5.     'api.my-website.com',
6.     // add third-parties
7.     'other-website.com',
8.     'example.com',
9.     // ...
10.   ]
11. });
12. ```

Allow all origins

Enables all cross-origin requests to be made.

Note: You may run into CORB and CORS issues!

0. ``` js
1. quicklink.listen({
2.   origins: true,
3.   // or
4.   origins: []
5. });
6. ```

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.

0. ``` js
1. // Same-origin restraint is enabled by default.
2. //
3. // This example will ignore all requests to:
4. //  - all "/api/*" pathnames
5. //  - all ".zip" extensions
6. //  - all tags with "noprefetch" attribute
7. //
8. quicklink.listen({
9.   ignores: [
10.     /\/api\/?/,
11.     uri => uri.includes('.zip'),
12.     (uri, elem) => elem.hasAttribute('noprefetch')
13.   ]
14. });
15. ```

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:

0. ``` js
1. quicklink.listen({
2.   ignores: [
3.     uri => uri.includes('#')
4.     // or RegExp: /#(.+)/
5.     // or element matching: (uri, elem) => !!elem.hash
6.   ]
7. });
8. ```

Custom URL to prefetch via hrefFn callback

The hrefFn method allows to build the URL to prefetch (e.g. API endpoint) on the fly instead of the prefetching the href attribute URL.

0. ``` js
1. quicklink.listen({
2.   hrefFn: function(element) {
3.     return element.href.replace('html','json');
4.   }
5. });
6. ```

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 bothSet() 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 (vianavigator.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:

0. ``` html
1. <script type="module">
2.   import { prefetch } from 'quicklink';
3.   prefetch(['1.html', '2.html']).catch(err => {
4.     // Handle own errors
5.   });
6. </script>
7. ```

Demo

Glitch demos

Using Quicklink in a multi-page site

Using Quicklink with Service Workers (via Workbox)

[Using Quicklink to prefetch API calls instead of href attribute](https://github.com/GoogleChromeLabs/quicklink/tree/master/demos/hrefFn)

Using Quicklink to prerender a specific page

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=<>×tamp=<>), 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 usesIntersection 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.

Magento 2 users can install the Quicklink Magento 2 module.

Want less aggressive prefetching? instant.page prefetches on mouseover and touchstart, right before a click.

License

Licensed under the Apache-2.0 license.