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    Drop-in replacement for react-ssr-prepass.

    Neither Preact nor React support Suspense on the server as of now. Heavily inspired by react-ssr-prepass, preact-ssr-prepass provides a two-pass approach with which Suspense can be used on the server. In the first pass, preact-ssr-prepass will create a VNode tree and await all suspensions, in the second pass preact-render-to-string can be used to render a vnode to a string.

    Even if preact-ssr-prepass is designed to do as little as possible, it still adds a slight overhead since the VNode tree is created twice.

    ⚠️ Note that this is neither an official Preact nor React API and that the way Suspense is handled on the server might/will change in the future!

    Usage / API

    Awaiting suspensions

    preact-ssr-prepass needs to be called just before rendering a vnode to a string. See the following example:


    export default function LazyLoaded() {
        return <div>I shall be loaded and rendered on the server</div>


    import { createElement as h } from 'preact';
    import { Suspense, lazy } from 'preact/compat';
    import renderToString from 'preact-render-to-string';
    import prepass from 'preact-ssr-prepass';
    const LazyComponent = lazy(() => import('./lazy'));
    const vnode = (
        <Suspense fallback={<div>I shall not be rendered on the server</div>}>
            <LazyComponent />
        .then(() => {
            // <div>I shall be loaded and rendered on the server</div>

    Custom suspensions/data fetching using the visitor

    preact-ssr-prepass accepts a second argument that allows you to suspend on arbitrary elements:

    ssrPrepass(<App />, (element, instance) => {
      if (instance !== undefined && typeof instance.fetchData === 'function') {
        return instance.fetchData()


     * Visitor function to suspend on certain elements.
     * When this function returns a Promise it is awaited before the vnode will be rendered.
    type Visitor = (element: preact.VNode, instance: ?preact.Component) => ?Promise<any>;
     * The default export of preact-ssr-prepass
     * @param{vnode} preact.VNode The vnode to traverse
     * @param{visitor} ?Visitor A function that is called for each vnode and might return a Promise to suspend.
     * @param{context} ?Object Initial context to be used when traversing the vnode tree
     * @return Promise<any> Promise that will complete once the complete vnode tree is traversed. Note that even if
     *         a Suspension throws the returned promise will resolve.
    export default function prepass(vnode: preact.VNode, visitor?: Visitor, context:? Object): Promise<any>;

    Replace react-ssr-prepass (e.g. next.js)

    react-ssr-prepass is usually used on the server only and not bundled into your bundles but rather required through Node.js. To alias react-ssr-prepass to preact-ssr-prepass we recommend to use module-alias:

    Create a file named alias.js:

    const moduleAlias = require('module-alias')
    module.exports = () => {
      moduleAlias.addAlias('react-ssr-prepass', 'preact-ssr-prepass')

    Require and execute the exported function in your applications entrypoint (before require'ing react-ssr-prepass):


    Differences to react-ssr-prepass

    The visitor passed to preact-ssr-prepass gets a Preact element instead of a React one. When you use preact/compat's createElement it will make the element/vnode look as similar to a React element as possible.




    npm i preact-ssr-prepass

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