Array.prototype.reduce

Reduce applies a function against an array, reducing the array to a single value.

Reducing is fairly simple, you might have previously done something like this...

var bits = ['a', 'b', 'c'];
var message = '';

for (var i = 0; i < bits.length; i++){
    message += bits[i];
}

message; // abc

This can be refactored to use reduce.

It's an array method and it takes 2 arguments, an accumulator function and an optional initialValue.

The accumulator function has 4 arguments, accumulator, currentValue, currentIndex & array.

accumulator

At first this is the initial value, then it becomes the previously returned value. In the below example it would be "a", then "ab", then "abc".

currentValue

This is simply the current value in the array, "a", "b" then "c".

currentIndex

This is just the index of the value in the array the accumulator is currently on. 0, 1 then 2.

array

This is the whole array reduce has been called on.

var bits = ['a', 'b', 'c'];

var message = bits.reduce(function(accumulator, currentValue, currentIndex, array) {
    return accumulator + currentValue;
}, []);

message; // abc

// or with an arrow function
var message = bits.reduce((a, b) => a + b);