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[2] + myarray[1] + myarray[0]; var result4 = myarray[3] + myarray[2] + myarray[1] + myarray[0]; 

so all that would output 5, 15, 18, 20

but instead of writing out all the vars like that, I want it to say something like:

var result = arrayitem + the sum of any previous items  

Does that make sense? Is that possible? How do I do that?

Answer 1

An elegant solution copied from Nina Scholz, using currying to access the previous value.

const cumulativeSum = (sum => value => sum += value)(0);    console.log([5, 10, 3, 2].map(cumulativeSum));

cumulativeSum is the function value => sum += value, with sum initialized to zero. Every time it's called, sum is updated and will equal the previous value (output[n-1]) when called the next time (with input[n]).

Answer 2

Javascript's reduce provides the current index, which is useful here:

var myarray = [5, 10, 3, 2]; var new_array = []; myarray.reduce(function(a,b,i) { return new_array[i] = a+b; },0); new_array // [5, 15, 18, 20]