What does the Reduce() JavaScript function do?

Question

I found a very useful function reduce(), and I\'m using it, but I\'m not sure if I understand it properly. Can anyone help me to understand this function?

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Answer 1

Taken from here, arr.reduce() will reduce the array to a value, specified by the callback. In your case, it will basically sum the elements of the array. Steps:

• Call function on 0,1 ( 0 is the initial value passed to .reduce() as the second argument. Return sum od 0 and 1, which is 1.
• Call function on previous result ( which is 1 ) and next array element. This returns sum of 1 and 2, which is 3
• Repeat until last element, which will sum up to 21

Answer 2

reduce() method has two parameters: a callback function that is called for every element in the array and an initial value.

The callback function also has two parameters: an accumulator value and the current value.

The flow for your array ([1, 2, 3, 4, 5, 6]) is like this:

1. return 0 + 1 // 0 is the accumulator which the first time takes the initial value, 1 is the current value. The result of this becomes the accumulator for the next call, and so on..
2. return 1 + 2 // 1 - accumulator, 2 - current value
3. return 3 + 3 // 3 - accumulator, 3 - current value, etc...
4. return 6 + 4
5. return 10 + 5
6. return 15 + 6

And when reached to the end of the array, return the accumulator, which here is 21