javascript - What does the Array method `reduce` 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?

Example:

var arr = [ 1, 2, 3, 4, 5, 6 ];
arr.reduce(function(p,n){
    return p + n;
}, 0);
// Output 21

This is my understanding: reduce loops through every element of the array and returns previous + current value. Ex. 0 + 1, 1 + 2 etc. In this case this function will return:

[0] - return 1
[1] - return 3
[2] - return 5
[3] - return 7
[4] - return 9
[5] - return 11

What next? Why does it give the result 21?

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