preload

preload("https://example.com/font.woff2", {as: "font"});

• Reference
  • preload(href, options)
• Usage
  • Preloading when rendering
  • Preloading in an event handler

Reference

preload(href, options)

To preload a resource, call the preload function from react-dom.

import { preload } from 'react-dom';

function AppRoot() {
  preload("https://example.com/font.woff2", {as: "font"});
  // ...
}

See more examples below.

The preload function provides the browser with a hint that it should start downloading the given resource, which can save time.

Parameters

• href: a string. The URL of the resource you want to download.
• options: an object. It contains the following properties:
  • as: a required string. The type of resource. Its possible values are audio, document, embed, fetch, font, image, object, script, style, track, video, worker.
  • crossOrigin: a string. The CORS policy to use. Its possible values are anonymous and use-credentials. It is required when as is set to "fetch".
  • referrerPolicy: a string. The Referrer header to send when fetching. Its possible values are no-referrer-when-downgrade (the default), no-referrer, origin, origin-when-cross-origin, and unsafe-url.
  • integrity: a string. A cryptographic hash of the resource, to verify its authenticity.
  • type: a string. The MIME type of the resource.
  • nonce: a string. A cryptographic nonce to allow the resource when using a strict Content Security Policy.
  • fetchPriority: a string. Suggests a relative priority for fetching the resource. The possible values are auto (the default), high, and low.
  • imageSrcSet: a string. For use only with as: "image". Specifies the source set of the image.
  • imageSizes: a string. For use only with as: "image". Specifies the sizes of the image.

Returns

preload returns nothing.

Caveats

• Multiple equivalent calls to preload have the same effect as a single call. Calls to preload are considered equivalent according to the following rules:
  • Two calls are equivalent if they have the same href, except:
  • If as is set to image, two calls are equivalent if they have the same href, imageSrcSet, and imageSizes.
• In the browser, you can call preload in any situation: while rendering a component, in an Effect, in an event handler, and so on.
• In server-side rendering or when rendering Server Components, preload only has an effect if you call it while rendering a component or in an async context originating from rendering a component. Any other calls will be ignored.

Usage

Preloading when rendering

Call preload when rendering a component if you know that it or its children will use a specific resource.

import { preload } from 'react-dom';

function AppRoot() {
  preload("https://example.com/script.js", {as: "script"});
  return ...;
}

Preloading in an event handler

Call preload in an event handler before transitioning to a page or state where external resources will be needed. This gets the process started earlier than if you call it during the rendering of the new page or state.

import { preload } from 'react-dom';

function CallToAction() {
  const onClick = () => {
    preload("https://example.com/wizardStyles.css", {as: "style"});
    startWizard();
  }
  return (
    <button onClick={onClick}>Start Wizard</button>
  );
}