var arr = [1,2,3,4,5,];

​

var result = arr.reduce(function(sum, nextValue) {

  return sum += nextValue;

});

1. first interation -- > sum(1) + next(2);

2. first interation -- > sum(3) + next(3);

3. first interation -- > sum(6) + next(4);

4. first interation -- > sum(10) + next(5) --> return 15;