simple cumulative sum in javascript.

cumsum.js

cumsum = [];
j = [0,1,2,3,4];

for(var a=0;a<j.length;a++) {
  if(a==0) cumsum[a] = j[0];
  else cumsum[a] = cumsum[a-1] + j[a];
}

With no branching:

var a  = [1,2,3,4,5];
for (var cumsum = [a[0]], i = 0, l = a.length-1; i<l; i++) 
    cumsum[i+1] = cumsum[i] + a[i+1]; 

@ghost's method does not work if you want to use let instead of var, because the let-variable gets discarded after the for-loop. Here's my alternative:

function cumSum(a) {
    let result = [a[0]];

    for(let i = 1; i < a.length; i++) {
      result[i] = result[i - 1] + a[i];
    }

    return result;
};

How about:

let y=0;
let b=[1,2,3,4,5];
cumsum = b.map(d=>y+=d);