parallel.js

import eachOf from './eachOf.js'
import _parallel from './internal/parallel.js'

/**
 * Run the `tasks` collection of functions in parallel, without waiting until
 * the previous function has completed. If any of the functions pass an error to
 * its callback, the main `callback` is immediately called with the value of the
 * error. Once the `tasks` have completed, the results are passed to the final
 * `callback` as an array.
 *
 * **Note:** `parallel` is about kicking-off I/O tasks in parallel, not about
 * parallel execution of code. If your tasks do not use any timers or perform
 * any I/O, they will actually be executed in series. Any synchronous setup
 * sections for each task will happen one after the other. JavaScript remains
 * single-threaded.
 *
 * **Hint:** Use [`reflect`]{@link module:Utils.reflect} to continue the
 * execution of other tasks when a task fails.
 *
 * It is also possible to use an object instead of an array. Each property will
 * be run as a function and the results will be passed to the final `callback`
 * as an object instead of an array. This can be a more readable way of handling
 * results from {@link async.parallel}.
 *
 * @name parallel
 * @static
 * @memberOf module:ControlFlow
 * @method
 * @category Control Flow
 * @param {Array|Iterable|AsyncIterable|Object} tasks - A collection of
 * [async functions]{@link AsyncFunction} to run.
 * Each async function can complete with any number of optional `result` values.
 * @param {Function} [callback] - An optional callback to run once all the
 * functions have completed successfully. This function gets a results array
 * (or object) containing all the result arguments passed to the task callbacks.
 * Invoked with (err, results).
 * @returns {Promise} a promise, if a callback is not passed
 *
 * @example
 *
 * //Using Callbacks
 * async.parallel([
 * function(callback) {
 * setTimeout(function() {
 * callback(null, 'one');
 * }, 200);
 * },
 * function(callback) {
 * setTimeout(function() {
 * callback(null, 'two');
 * }, 100);
 * }
 * ], function(err, results) {
 * console.log(results);
 * // results is equal to ['one','two'] even though
 * // the second function had a shorter timeout.
 * });
 *
 * // an example using an object instead of an array
 * async.parallel({
 * one: function(callback) {
 * setTimeout(function() {
 * callback(null, 1);
 * }, 200);
 * },
 * two: function(callback) {
 * setTimeout(function() {
 * callback(null, 2);
 * }, 100);
 * }
 * }, function(err, results) {
 * console.log(results);
 * // results is equal to: { one: 1, two: 2 }
 * });
 *
 * //Using Promises
 * async.parallel([
 * function(callback) {
 * setTimeout(function() {
 * callback(null, 'one');
 * }, 200);
 * },
 * function(callback) {
 * setTimeout(function() {
 * callback(null, 'two');
 * }, 100);
 * }
 * ]).then(results => {
 * console.log(results);
 * // results is equal to ['one','two'] even though
 * // the second function had a shorter timeout.
 * }).catch(err => {
 * console.log(err);
 * });
 *
 * // an example using an object instead of an array
 * async.parallel({
 * one: function(callback) {
 * setTimeout(function() {
 * callback(null, 1);
 * }, 200);
 * },
 * two: function(callback) {
 * setTimeout(function() {
 * callback(null, 2);
 * }, 100);
 * }
 * }).then(results => {
 * console.log(results);
 * // results is equal to: { one: 1, two: 2 }
 * }).catch(err => {
 * console.log(err);
 * });
 *
 * //Using async/await
 * async () => {
 * try {
 * let results = await async.parallel([
 * function(callback) {
 * setTimeout(function() {
 * callback(null, 'one');
 * }, 200);
 * },
 * function(callback) {
 * setTimeout(function() {
 * callback(null, 'two');
 * }, 100);
 * }
 * ]);
 * console.log(results);
 * // results is equal to ['one','two'] even though
 * // the second function had a shorter timeout.
 * }
 * catch (err) {
 * console.log(err);
 * }
 * }
 *
 * // an example using an object instead of an array
 * async () => {
 * try {
 * let results = await async.parallel({
 * one: function(callback) {
 * setTimeout(function() {
 * callback(null, 1);
 * }, 200);
 * },
 * two: function(callback) {
 * setTimeout(function() {
 * callback(null, 2);
 * }, 100);
 * }
 * });
 * console.log(results);
 * // results is equal to: { one: 1, two: 2 }
 * }
 * catch (err) {
 * console.log(err);
 * }
 * }
 *
 */
export default function parallel(tasks, callback) {
    return _parallel(eachOf, tasks, callback);
}