Swup Preload Plugin

A swup plugin for preloading pages and faster navigation.

• Links with a data-swup-preload attribute will be preloaded automatically
• Hovering a link on pointer devices will preload that link's URL, speeding up load time by a few 100ms. To save server resources, the number of simultaneous preload requests is limited to 5 by default.
• Touch devices will instead preload links at the start of touch events, giving a ~80ms speed-up
• Optionally preload links as they become visible in the viewport

Installation

Install the plugin from npm and import it into your bundle.

npm install @swup/preload-plugin

import SwupPreloadPlugin from '@swup/preload-plugin';

Or include the minified production file from a CDN:

<script src="https://unpkg.com/@swup/preload-plugin@3"></script>

Usage

To run this plugin, include an instance in the swup options.

const swup = new Swup({
  plugins: [new SwupPreloadPlugin()]
});

Preloading

The plugin supports four ways of preloading links:

• Hovering a link
• Marking links to preload with a special attribute
• Watching the viewport for links to become visible
• Passing in a list of URLs to preload at once

Hovering links

Hovering a link will automatically preload it. Enabled by default.

Depending on the user's device, the preload will be triggered when it is hovered with a mouse, touched with a finger, or focused using the keyboard. Hovered links are preloaded with higher priority than other running requests.

<a href="/about">About</a> <!-- will preload when hovering -->

Marking links to preload

To preload specific links, mark them with the data-swup-preload attribute.

<a href="/about" data-swup-preload>About</a>

To preload all links in a container, mark the container with data-swup-preload-all.

<nav data-swup-preload-all>
  <a href="/about">About</a>
  <a href="/contact">Contact</a>
</nav>

Preload links as they become visible

Preload links as they enter the viewport. Not enabled by default.

See the preloadVisibleLinks option for details.

Preload a list of URLs

Preload specific known URLs.

See the swup.preload() method for details.

Options

throttle

Type: Number, Default: 5

The concurrency limit for simultaneous requests when preloading.

preloadHoveredLinks

Type: Boolean, Default: true

Preload links when they are hovered, touched or focused.

preloadVisibleLinks

Type: Boolean | Object, Default: false

Preload links when they enter the viewport. Pass in a boolean true to enable with default options.

new SwupPreloadPlugin({ preloadVisibleLinks: true })

For more control over the behavior, pass in an object. These are the default options:

new SwupPreloadPlugin({
  preloadVisibleLinks: {
    /** How much area of a link must be visible to preload it: 0 to 1.0 */
    threshold: 0.2,
    /** How long a link must be visible to preload it, in milliseconds */
    delay: 500,
    /** Containers to look for links in */
    containers: ['body'],
    /** Callback for opting out selected elements from preloading */
    ignore: (el) => false
  }
})

preloadInitialPage

Type: Boolean, Default: true

Preload the initial page to allow instant back-button navigation after having navigated away from it. Disable this if it causes issues or doesn't make sense in your specific scenario.

Methods on the swup instance

The plugin adds two methods for preloading pages to the swup instance.

preload

Preload a URL or array of URLs. Returns a Promise that resolves when all requested pages have been preloaded.

await swup.preload('/path/to/page');
await swup.preload(['/some/page', '/other/page']);

preloadLinks

Scan the DOM for links with the attribute [data-swup-preload] and call preload for each URL:

swup.preloadLinks();

Hooks

The plugin creates two new hooks.

Note The visit object might be undefined or already settled for these hooks

page:preload

Fires when a page was preloaded.

swup.hooks.on('page:preload', (_visit, { page }) => console.log('preloaded:', page));

link:hover

Fires every time a link is hovered.

swup.hooks.on('link:hover', (_visit, { el }) => console.log('link hovered:', el));