Creating an array of cumulative sum in javascript

Question

This is an example of what I need to do:

var myarray = [5, 10, 3, 2];

var result1 = myarray[0];
var result2 = myarray[1] + myarray[0];
var result3 = myarray         


        

Answer 1

My initial ES6 thought was similar to a few above answers by Taeho and others.

const cumulativeSum = ([head, ...tail]) =>
   tail.reduce((acc, x, index) => {
      acc.push(acc[index] + x);
      return acc
  }, [head])
console.log(cumulativeSum([-1,2,3])

The solution performs:

n lookups, n - 1 sums and 0 conditional evaluations

Most of what I saw above appeared to use:

n lookups, 2n sums, and n conditional evaluations:

You could do this with ie6 safe js as well. This is possibly more efficient since you don't have to create the tail spread array.

function cumulativeSum(a) {
    var result = [a[0]];
    var last = a[0];
    for (i = 1; i < a.length; i++) {
        last = last + a[i];
        result.push(last)
    }
    return result;
}
console.log(cumulativeSum([-1,2,3]))