preinit

Note

React-based frameworks frequently handle resource loading for you, so you might not have to call this API yourself. Consult your framework’s documentation for details.

preinit lets you eagerly fetch and evaluate a stylesheet or external script.

preinit("https://example.com/script.js",{as:"script"});

Reference

`preinit(href, options)`

To preinit a script or stylesheet, call thepreinit function fromreact-dom.

import{preinit}from'react-dom';

functionAppRoot(){
preinit("https://example.com/script.js",{as:"script"});
// ...
}

See more examples below.

Thepreinit function provides the browser with a hint that it should start downloading and executing the given resource, which can save time. Scripts that youpreinit are executed when they finish downloading. Stylesheets that you preinit are inserted into the document, which causes them to go into effect right away.

Parameters

href: a string. The URL of the resource you want to download and execute.
options: an object. It contains the following properties:
- as: a required string. The type of resource. Its possible values arescript andstyle.
- precedence: a string. Required with stylesheets. Says where to insert the stylesheet relative to others. Stylesheets with higher precedence can override those with lower precedence. The possible values arereset,low,medium,high.
- crossOrigin: a string. TheCORS policy to use. Its possible values areanonymous anduse-credentials. It is required whenas is set to"fetch".
- integrity: a string. A cryptographic hash of the resource, toverify its authenticity.
- nonce: a string. A cryptographicnonce 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 areauto (the default),high, andlow.

Returns

preinit returns nothing.

Caveats

Multiple calls topreinit with the samehref have the same effect as a single call.
In the browser, you can callpreinit 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,preinit 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

Preiniting when rendering

Callpreinit when rendering a component if you know that it or its children will use a specific resource, and you’re OK with the resource being evaluated and thereby taking effect immediately upon being downloaded.

Examples of preiniting

Example 1 of 2:
Preiniting an external script

import{preinit}from'react-dom';

functionAppRoot(){
preinit("https://example.com/script.js",{as:"script"});
return...;
}

If you want the browser to download the script but not to execute it right away, usepreload instead. If you want to load an ESM module, usepreinitModule.

Preiniting in an event handler

Callpreinit 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{preinit}from'react-dom';

functionCallToAction(){
constonClick =()=>{
preinit("https://example.com/wizardStyles.css",{as:"style"});
startWizard();
}
return(
<buttononClick={onClick}>Start Wizard</button>
);
}

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