Скачать или смотреть [ES6] reduce

[ES6] reduce

The reduce() method executes a reducer function (that you provide) on each element of the array, resulting in single output value.

The reduce() method executes the callback once for each assigned value present in the array, taking four arguments:
• accumulator
• currentValue
• currentIndex
• array

The first time the callback is called, accumulator and currentValue can be one of two values. If initialValue is provided in the call to reduce(), then the accumulator will be equal to initialValue, and currentValue will be equal to the first value in the array. If no initialValue is provided, then the accumulator will be equal to the first value in the array, and currentValue will be equal to the second.

If the array is empty and no initialValue is provided, TypeError will be thrown. If the array only has one element (regardless of position) and no initialValue is provided, or if initialValue is provided but the array is empty, the solo value will be returned without calling the callback.

It is usually safer to provide an initialValue because there are three possible outputs without initialValue.