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];
}

How about:

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