# 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](https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API))
* **Waits until the browser is idle** (using [requestIdleCallback](https://developer.mozilla.org/en-US/docs/Web/API/Window/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 [``](https://www.w3.org/TR/resource-hints/#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](http://nodejs.org) and [npm](https://npmjs.com):
```sh
npm install --save quicklink
```
You can also grab `quicklink` from [unpkg.com/quicklink](https://unpkg.com/quicklink).
## Usage
Once initialized, `quicklink` will automatically prefetch URLs for links that are in-viewport during idle time.
Quickstart:
```html
```
For example, you can initialize after the `load` event fires:
```html
```
ES Module import:
```js
import quicklink from "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](https://github.com/pastelsky/network-idle-callback) (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)
TODO:
* Explore detecting file-extension of resources and using [rel=preload](https://w3c.github.io/preload/) for high priority fetches
* Explore using [Priority Hints](https://github.com/WICG/priority-hints) for importance hinting
## 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 [CanIUse](https://caniuse.com/#feat=intersectionobserver)). We recommend conditionally polyfilling this feature with a service like Polyfill.io:
```html
```
Alternatively, see the [Intersection Observer polyfill](https://github.com/w3c/IntersectionObserver/tree/master/polyfill).
## Recipes
**Set a custom timeout for prefetching resources**
Defaults to 2 seconds (via `requestIdleCallback`). Here we override it to 4 seconds:
```js
quicklink({
timeout: 4000
});
```
**Set the DOM element to observe for in-viewport links**
Defaults to `document` otherwise.
```js
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.
```js
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.
```js
quicklink({ priority: true });
```
## 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, Firefox, Edge, Opera, Android Browser, Samsung Internet.
* With [Intersection Observer polyfill](https://github.com/w3c/IntersectionObserver/tree/master/polyfill) ~6KB gzipped/minified: Safari, IE9+
Certain features have layered support. If opting for `{priority: true}` and `fetch()` 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:
```html
```
## Demo
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.
## 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/optimize-prefetching-with-guessjs/).
## License
Licensed under the Apache-2.0 license.
