The `preload` Extension

The preload extension allows you to load HTML fragments into your browser’s cache before they are requested by the user, so that additional pages appear to users to load nearly instantaneously. As a developer, you can customize its behavior to fit your applications needs and use cases.

IMPORTANT: Preloading content judiciously can improve your web application’s perceived performance, but preloading too many resources can negatively impact your visitors’ bandwidth and your server performance by initiating too many unused requests. Use this extension carefully!

#Install

<script src="https://unpkg.com/htmx.org/dist/ext/preload.js"></script>

#Usage

Register the extension with htmx using the hx-ext attribute. Then, add a preload attribute to any hyperlinks and hx-get elements you want to preload. By default, resources will be loaded as soon as the mousedown event begins, giving your application a roughly 100-200ms head start on serving responses. See configuration below for other options.

<body hx-ext="preload">
    <h1>What Works</h2>
    <a href="/server/1" preload>WILL BE requested using a standard XMLHttpRequest() and default options (below)</a>
    <button hx-get="/server/2" preload>WILL BE requested with additional htmx headers.</button>

    <h1>What WILL NOT WORK</h1>
    <a href="/server/3">WILL NOT be preloaded because it does not have an explicit "preload" attribute</a>
    <a hx-post="/server/4" preload>WILL NOT be preloaded because it is an HX-POST transaction.</a>
</body>

#Inheriting Preload Settings

You can add the preload attribute to the top-level element that contains several <a href=""> or hx-get="" elements, and all of them will be preloaded. Be careful with this setting, because you can end up wasting bandwidth if you preload many more resources than you need.

<body hx-ext="preload">
    <ul preload>
        <li><a href="/server/1">This will be preloaded because of the attribute in the node above.</a>
        <li><a href="/server/2">This will also be preloaded for the same reason.</a>
        <li><a href="/server/3">This will be preloaded, too.  Lorem ipsum.</a>
    </ul>
</body>

#Preloading of Linked Images

After an HTML page (or page fragment) is preloaded, this extension can also preload linked image resources. It will not load or run linked Javascript or Cascading Stylesheet content, whether linked or embedded in the preloaded HTML. To preload images as well, use the following syntax.

<div hx-ext="preload">
    <a href="/my-next-page" preload="mouseover" preload-images="true">Next Page</a>
</div>

#Configuration

Defaults for this extension are chosen to balance users’ perceived performance with potential load on your servers from unused requests. As a developer, you can modify two settings to customize this behavior to your specific use cases.

#preload=“mousedown” (DEFAULT)

The default behavior for this extension is to begin loading a resource when the user presses the mouse down. This is a conservative setting that guarantees the user actually intends to use the linked resource. Because user click events typically take 100-200ms to complete, this setting gives your server a significant headstart compared with a regular click.

<a href="/server/1" preload="mousedown">This will be preloaded when the user begins to click.</a>

#preload=“mouseover”

To preload links more aggressively, you can trigger the preload to happen when the user’s mouse hovers over the link instead. To prevent many resources from being loaded when the user scrolls or moves the mouse across a large list of objects, a 100ms delay is built in to this action. If the user’s mouse leaves the element before this timeout expires, then the resource is not preloaded.

Typical users hover over links for several hundred milliseconds before they click, which gives your server even more time to respond to the request than the mousedown option above. Test your own hover timing here.. However, be careful when using this option because it can increase server load by requesting resources unnecessarily.

<a href="/server/1" preload="mouseover">This will be preloaded when the user's mouse remains over it for more than 100ms.</a>

#preload=“custom-event-name”

Preload can also listen to any custom event within the system, triggering resources to be preloaded (if they have not already been cached by the browser). The extension itself generates an event called preload:init that can be used to trigger preloads as soon as an object has been processed by htmx.

<body hx-ext="preload">
    <button hx-get="/server" preload="preload:init" hx-target="idLoadMore">Load More</a>
    <div id="idLoadMore">
        Content for this DIV will be preloaded as soon as the page is ready.
        Clicking the button above will swap it into the DOM.
    </div>
</body>

#About Touch Events

To accommodate touchscreen devices, an additional ontouchstart event handler is added whenever you specify a mouseover or mousedown trigger. This extra trigger fires immediately (no waiting period) whenever the user touches the screen, saving you 300ms of waiting time on Android, and 450ms on iOS.

#Limitations

Links must be marked with a preload attribute, or have an ancestor node that has the preload attribute.
Only GET transactions (including <a href=""> and hx-get="") can be preloaded. Following REST principles, GET transactions are assumed to not make any significant changes to a resource. Transactions that can potentially make a change (such as POST, PUT, and DELETE) will not be preloaded under any circumstances.
When listening to mouseover events, preload waits for 100ms before downloading the linked resource. If the mouse leaves the resource before this timeout expires, the resource is not preloaded.
Preloaded responses will only be cached in the browser if the response headers allow it. For example, the response header Cache-Control: private, max-age=60 allows the browser to cache the response, whereas Cache-Control: no-cache prevents it.

#Credits

The behavior for this plugin was inspired by the work done by Alexandre Dieulot on InstantClick, which is released under the MIT license.

The preload Extension