Cumulative sum

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]).

Note that sum will need to be set to zero explicitly when you want to reuse the summation. The most convenient way to handle this may be to just inline the code instead of using a named function.

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