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import eachOfSeries from './eachOfSeries';
/**
* Reduces `coll` into a single value using an async `iteratee` to return each
* successive step. `memo` is the initial state of the reduction. This function
* only operates in series.
*
* For performance reasons, it may make sense to split a call to this function
* into a parallel map, and then use the normal `Array.prototype.reduce` on the
* results. This function is for situations where each step in the reduction
* needs to be async; if you can get the data before reducing it, then it's
* probably a good idea to do so.
*
* @name reduce
* @static
* @memberOf async
* @alias inject
* @alias foldl
* @category Collection
* @param {Array|Object} coll - A collection to iterate over.
* @param {*} memo - The initial state of the reduction.
* @param {Function} iteratee - A function applied to each item in the
* array to produce the next step in the reduction. The `iteratee` is passed a
* `callback(err, reduction)` which accepts an optional error as its first
* argument, and the state of the reduction as the second. If an error is
* passed to the callback, the reduction is stopped and the main `callback` is
* immediately called with the error. Invoked with (memo, item, callback).
* @param {Function} [callback] - A callback which is called after all the
* `iteratee` functions have finished. Result is the reduced value. Invoked with
* (err, result).
* @example
*
* async.reduce([1,2,3], 0, function(memo, item, callback) {
* // pointless async:
* process.nextTick(function() {
* callback(null, memo + item)
* });
* }, function(err, result) {
* // result is now equal to the last value of memo, which is 6
* });
*/
export default function reduce(coll, memo, iteratee, callback) {
eachOfSeries(coll, function(x, i, callback) {
iteratee(memo, x, function(err, v) {
memo = v;
callback(err);
});
}, function(err) {
callback(err, memo);
});
}
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