transform.js

import eachOf from './eachOf';
import once from './internal/once';
import wrapAsync from './internal/wrapAsync';
import { promiseCallback, PROMISE_SYMBOL } from './internal/promiseCallback';

/**
 * A relative of `reduce`.  Takes an Object or Array, and iterates over each
 * element in parallel, each step potentially mutating an `accumulator` value.
 * The type of the accumulator defaults to the type of collection passed in.
 *
 * @name transform
 * @static
 * @memberOf module:Collections
 * @method
 * @category Collection
 * @param {Array|Iterable|AsyncIterable|Object} coll - A collection to iterate over.
 * @param {*} [accumulator] - The initial state of the transform.  If omitted,
 * it will default to an empty Object or Array, depending on the type of `coll`
 * @param {AsyncFunction} iteratee - A function applied to each item in the
 * collection that potentially modifies the accumulator.
 * Invoked with (accumulator, item, key, callback).
 * @param {Function} [callback] - A callback which is called after all the
 * `iteratee` functions have finished. Result is the transformed accumulator.
 * Invoked with (err, result).
 * @returns {Promise} a promise, if no callback provided
 * @example
 *
 * async.transform([1,2,3], function(acc, item, index, callback) {
 *     // pointless async:
 *     process.nextTick(function() {
 *         acc[index] = item * 2
 *         callback(null)
 *     });
 * }, function(err, result) {
 *     // result is now equal to [2, 4, 6]
 * });
 *
 * @example
 *
 * async.transform({a: 1, b: 2, c: 3}, function (obj, val, key, callback) {
 *     setImmediate(function () {
 *         obj[key] = val * 2;
 *         callback();
 *     })
 * }, function (err, result) {
 *     // result is equal to {a: 2, b: 4, c: 6}
 * })
 */
export default function transform (coll, accumulator, iteratee, callback) {
    if (arguments.length <= 3 && typeof accumulator === 'function') {
        callback = iteratee;
        iteratee = accumulator;
        accumulator = Array.isArray(coll) ? [] : {};
    }
    callback = once(callback || promiseCallback());
    var _iteratee = wrapAsync(iteratee);

    eachOf(coll, (v, k, cb) => {
        _iteratee(accumulator, v, k, cb);
    }, err => callback(err, accumulator));
    return callback[PROMISE_SYMBOL]
}