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/**
* A helper function for sequencing multiple `handle` calls in a middleware-like manner.
* The behavior for the `handle` options is as follows:
* - `transformPageChunk` is applied in reverse order and merged
* - `preload` is applied in forward order, the first option "wins" and no `preload` options after it are called
* - `filterSerializedResponseHeaders` behaves the same as `preload`
*
* ```js
* /// file: src/hooks.server.js
* import { sequence } from '@sveltejs/kit/hooks';
*
* /// type: import('@sveltejs/kit').Handle
* async function first({ event, resolve }) {
* console.log('first pre-processing');
* const result = await resolve(event, {
* transformPageChunk: ({ html }) => {
* // transforms are applied in reverse order
* console.log('first transform');
* return html;
* },
* preload: () => {
* // this one wins as it's the first defined in the chain
* console.log('first preload');
* }
* });
* console.log('first post-processing');
* return result;
* }
*
* /// type: import('@sveltejs/kit').Handle
* async function second({ event, resolve }) {
* console.log('second pre-processing');
* const result = await resolve(event, {
* transformPageChunk: ({ html }) => {
* console.log('second transform');
* return html;
* },
* preload: () => {
* console.log('second preload');
* },
* filterSerializedResponseHeaders: () => {
* // this one wins as it's the first defined in the chain
* console.log('second filterSerializedResponseHeaders');
* }
* });
* console.log('second post-processing');
* return result;
* }
*
* export const handle = sequence(first, second);
* ```
*
* The example above would print:
*
* ```
* first pre-processing
* first preload
* second pre-processing
* second filterSerializedResponseHeaders
* second transform
* first transform
* second post-processing
* first post-processing
* ```
*
* @param {...import('@sveltejs/kit').Handle} handlers The chain of `handle` functions
* @returns {import('@sveltejs/kit').Handle}
*/
export function sequence(...handlers) {
const length = handlers.length;
if (!length) return ({ event, resolve }) => resolve(event);
return ({ event, resolve }) => {
return apply_handle(0, event, {});
/**
* @param {number} i
* @param {import('@sveltejs/kit').RequestEvent} event
* @param {import('@sveltejs/kit').ResolveOptions | undefined} parent_options
* @returns {import('types').MaybePromise<Response>}
*/
function apply_handle(i, event, parent_options) {
const handle = handlers[i];
return handle({
event,
resolve: (event, options) => {
/** @type {import('@sveltejs/kit').ResolveOptions['transformPageChunk']} */
const transformPageChunk = async ({ html, done }) => {
if (options?.transformPageChunk) {
html = (await options.transformPageChunk({ html, done })) ?? '';
}
if (parent_options?.transformPageChunk) {
html = (await parent_options.transformPageChunk({ html, done })) ?? '';
}
return html;
};
/** @type {import('@sveltejs/kit').ResolveOptions['filterSerializedResponseHeaders']} */
const filterSerializedResponseHeaders =
parent_options?.filterSerializedResponseHeaders ??
options?.filterSerializedResponseHeaders;
/** @type {import('@sveltejs/kit').ResolveOptions['preload']} */
const preload = parent_options?.preload ?? options?.preload;
return i < length - 1
? apply_handle(i + 1, event, {
transformPageChunk,
filterSerializedResponseHeaders,
preload
})
: resolve(event, { transformPageChunk, filterSerializedResponseHeaders, preload });
}
});
}
};
}
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