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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]
}
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