The DOM element to observe for in-viewport links to prefetch or the NodeList of Anchor Elements.

#### options.limit

- Type: `Number`
- Default: `Infinity`

The _total_ requests that can be prefetched or prerendered while observing the `options.el` container.

#### options.threshold

- Type: `Number`
- Default: `0`

The _area percentage_ of each link that must have entered the viewport to be fetched, in its decimal form (e.g. 0.25 = 25%).

#### 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](https://github.com/pastelsky/network-idle-callback) (see demos).

By default, this uses [`requestIdleCallback`](https://developer.mozilla.org/en-US/docs/Web/API/Window/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`
- 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](https://developer.mozilla.org/en-US/docs/Web/API/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:** Speculative Rules API supports same-site cross origin Prerendering with [opt-in header](https://bit.ly/ss-cross-origin-pre).

## Polyfills

`quicklink`:

- Includes a very small fallback for [requestIdleCallback](https://developer.mozilla.org/en-US/docs/Web/API/Window/requestIdleCallback)
- Requires `IntersectionObserver` to be supported (see [Can I Use](https://caniuse.com/intersectionobserver)). We recommend conditionally polyfilling this feature with a service like Polyfill.io:

```html

```

Alternatively, see the [Intersection Observer polyfill](https://github.com/GoogleChromeLabs/intersection-observer).

## Recipes

### Set a custom timeout for prefetching resources

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

```js
quicklink.listen({
timeout: 4000,
});
```

### Set a specific Anchor Elements NodeList to observe for in-viewport links

Defaults to `document` otherwise.

```js
quicklink.listen({
el: document.querySelectorAll('a.linksToPrefetch'),
});
```

### Set the DOM element to observe for in-viewport links

Defaults to `document` otherwise.

```js
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.

```js
// 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);
```

### 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.

```js
// Single URL
quicklink.prerender('2.html');

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

### 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.

```js
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!

```js
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](https://chromium.googlesource.com/chromium/src/+/main/services/network/cross_origin_read_blocking_explainer.md) and [CORS](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS) issues!

```js
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.

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:

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

### 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.

```js
quicklink.listen({
hrefFn(element) {
return element.href.replace('html', 'json');
},
});
```

## Browser Support

The prefetching provided by `quicklink` can be viewed as a [progressive enhancement](https://www.smashingmagazine.com/2009/04/progressive-enhancement-what-it-is-and-how-to-use-it/). Cross-browser support is as follows:

- Without polyfills: Chrome, Safari ≥ 12.1, Firefox, Edge, Opera, Android Browser, Samsung Internet.
- With [Intersection Observer polyfill](https://github.com/GoogleChromeLabs/intersection-observer) ~6KB gzipped/minified: Safari ≤ 12.0, IE11
- With the above and a [Set()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set) and [Array.from](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/from) polyfill: IE9 and IE10. [Core.js](https://github.com/zloirock/core-js) provides both `Set()` and `Array.from()` shims. Projects like [es6-shim](https://github.com/paulmillr/es6-shim/blob/master/README.md) are an alternative you can consider.

Certain features have layered support:

- The [Network Information API](https://wicg.github.io/netinfo/), 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+](https://caniuse.com/netinfo)
- If opting for `{priority: true}` and the [Fetch API](https://fetch.spec.whatwg.org/) 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:

```html

import {prefetch} from 'quicklink';
prefetch(['1.html', '2.html']).catch(error => {
// Handle own errors
});

```

## Demo

### Glitch demos

- [Using Quicklink in a multi-page site](https://github.com/GoogleChromeLabs/quicklink/tree/main/demos/news)
- [Using Quicklink with Service Workers (via Workbox)](https://github.com/GoogleChromeLabs/quicklink/tree/main/demos/news-workbox)
- [Using Quicklink to prefetch API calls instead of `href` attribute](https://github.com/GoogleChromeLabs/quicklink/tree/main/demos/hrefFn)
- [Using Quicklink to prerender a specific page](https://uskay-prerender2.glitch.me/next.html)

### Research

Here's a [WebPageTest run](https://www.webpagetest.org/video/view.php?id=181212_4c294265117680f2636676721cc886613fe2eede&data=1) for our [demo](https://keyword-2-ecd7b.firebaseapp.com/) improving page-load performance by up to 4 seconds via quicklink's prefetching. A [video](https://youtu.be/rQ75YEbJicw) comparison of the before/after prefetching is on YouTube.

For demo purposes, we deployed a version of the [Google Blog](https://blog.google) 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](https://caniuse.com/ping) (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](#custom-ignore-patterns).

## Related projects

- Using [Gatsby](https://gatsbyjs.org)? 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](https://guess-js.github.io). It uses analytics and machine-learning to prefetch resources based on how users navigate your site. It also has plugins for [Webpack](https://www.npmjs.com/package/guess-webpack) and [Gatsby](https://www.gatsbyjs.org/docs/optimizing-site-performance-with-guessjs/).
- WordPress users can now get quicklink as a [WordPress Plugin from the plugin repository](https://wordpress.org/plugins/quicklink/).
- Drupal users can install the [Quicklink Drupal module](https://www.drupal.org/project/quicklink).
- Magento 2 users can install the [rafaelcg-magento2-quicklink](https://marketplace.magento.com/rafaelcg-magento2-quicklink.html) or [rangerz/magento2-module-quicklink](https://github.com/rangerz/magento2-module-quicklink).
- Want less aggressive prefetching? [instant.page](https://instant.page/) prefetches on mouseover and touchstart, right before a click.

## License