d3.cumsum?

[yurivish]

Taking a cumulative sum is a common function in numeric computing and I've found myself repeatedly reaching for it over the years.

[1, 2, 3, 4].reduce((a, b) => (a.push(b + (a.length ? a[a.length-1] : 0)), a), [])

It can be implemented in one line, though you can achieve better performance by copying the array and running through 1:n adding a[n-1] to a[n] inside each iteration.

[mbostock]

I think I would implement this as

function cumsum(values) {
  let sum = 0;
  return Float64Array.from(values, v => sum += v);
}

so that it works with iterables. But I’d also change it to coerce to numbers, treat NaN as zero, and take an accessor.

[Fil]

function cumsum(values, valueof) {
  var sum = 0, index = 0, value;
  return Float64Array.from(values, valueof === undefined
    ? v => (sum += +v || 0)
    : v => (sum += +valueof(v, index++, values) || 0));
}

(edited with Mike's suggestion)
(re-edited with index++ rather than ++index, so that valueof(d,i) starts with i=0)

https://observablehq.com/@fil/cumsum

[Fil]

Documentation: https://observablehq.com/d/918b60f6963a81e1

[mbostock]

Added in 2.4.