webpack/lib/util/processAsyncTree.js

Packs ECMAScript/CommonJs/AMD modules for the browser. Allows you to split your codebase into multiple bundles, which can be loaded on demand. Supports loaders to preprocess files, i.e. json, jsx, es7, css, less, ... and your custom stuff.

/*
	MIT License http://www.opensource.org/licenses/mit-license.php
	Author Tobias Koppers @sokra
*/

"use strict";

/**
 * Walks a dynamically expanding async work tree with bounded concurrency.
 * Each processed item may enqueue more items through `push`, allowing callers
 * to model breadth-first or depth-first discovery without managing the queue
 * themselves.
 * @template T
 * @template {Error} E
 * @param {Iterable<T>} items initial items
 * @param {number} concurrency number of items running in parallel
 * @param {(item: T, push: (item: T) => void, callback: (err?: E) => void) => void} processor worker which pushes more items
 * @param {(err?: E) => void} callback all items processed
 * @returns {void}
 */
const processAsyncTree = (items, concurrency, processor, callback) => {
	const queue = [...items];
	if (queue.length === 0) return callback();
	let processing = 0;
	let finished = false;
	let processScheduled = true;

	/**
	 * Enqueues a newly discovered item and schedules queue processing when the
	 * current concurrency budget allows more work to start.
	 * @param {T} item item
	 */
	const push = (item) => {
		queue.push(item);
		if (!processScheduled && processing < concurrency) {
			processScheduled = true;
			process.nextTick(processQueue);
		}
	};

	/**
	 * Handles completion of a single processor call, propagating the first
	 * error and scheduling more queued work when possible.
	 * @param {E | null | undefined} err error
	 */
	const processorCallback = (err) => {
		processing--;
		if (err && !finished) {
			finished = true;
			callback(err);
			return;
		}
		if (!processScheduled) {
			processScheduled = true;
			process.nextTick(processQueue);
		}
	};

	const processQueue = () => {
		if (finished) return;
		while (processing < concurrency && queue.length > 0) {
			processing++;
			const item = /** @type {T} */ (queue.pop());
			processor(item, push, processorCallback);
		}
		processScheduled = false;
		if (queue.length === 0 && processing === 0 && !finished) {
			finished = true;
			callback();
		}
	};

	processQueue();
};

module.exports = processAsyncTree;