Create an array of partial sums in JavaScript

The partial sum of an array is the sum of all elements up to a certain index. While you may instinctively reach for Array.prototype.map() to calculate partial sums, you'll find that you need to keep track of the previous sum. This is where Array.prototype.reduce() comes in handy.

Using Array.prototype.reduce() allows you to keep track of the previous sum while iterating over the array. By initializing the accumulator with an empty array, you can then use Array.prototype.slice() to get the previous partial sum or 0 if it's the first element.

Finally, using the spread operator (...), you can add the new partial sum to the accumulator array containing the previous sums.

const accumulate = (...nums) =>
  nums.reduce((acc, n) => [...acc, n + (acc.slice(-1)[0] || 0)], []);

accumulate(1, 2, 3, 4); // [1, 3, 6, 10]
accumulate(...[1, 2, 3, 4]); // [1, 3, 6, 10]