@sveltejs/kit/src/exports/hooks/sequence.js

SvelteKit is the fastest way to build Svelte apps

/** @import { Handle, RequestEvent, ResolveOptions } from '@sveltejs/kit' */
/** @import { MaybePromise } from 'types' */
import {
	merge_tracing,
	get_request_store,
	with_request_store
} from '@sveltejs/kit/internal/server';

/**
 * 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');
 * 			return true;
 * 		}
 * 	});
 * 	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');
 * 			return true;
 * 		},
 * 		filterSerializedResponseHeaders: () => {
 * 			// this one wins as it's the first defined in the chain
 * 			console.log('second filterSerializedResponseHeaders');
 * 			return true;
 * 		}
 * 	});
 * 	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 {...Handle} handlers The chain of `handle` functions
 * @returns {Handle}
 */
export function sequence(...handlers) {
	const length = handlers.length;
	if (!length) return ({ event, resolve }) => resolve(event);

	return ({ event, resolve }) => {
		const { state } = get_request_store();
		return apply_handle(0, event, {});

		/**
		 * @param {number} i
		 * @param {RequestEvent} event
		 * @param {ResolveOptions | undefined} parent_options
		 * @returns {MaybePromise<Response>}
		 */
		function apply_handle(i, event, parent_options) {
			const handle = handlers[i];

			return state.tracing.record_span({
				name: `sveltekit.handle.sequenced.${handle.name ? handle.name : i}`,
				attributes: {},
				fn: async (current) => {
					const traced_event = merge_tracing(event, current);
					return await with_request_store({ event: traced_event, state }, () =>
						handle({
							event: traced_event,
							resolve: (event, options) => {
								/** @type {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 {ResolveOptions['filterSerializedResponseHeaders']} */
								const filterSerializedResponseHeaders =
									parent_options?.filterSerializedResponseHeaders ??
									options?.filterSerializedResponseHeaders;

								/** @type {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
										});
							}
						})
					);
				}
			});
		}
	};
}