Simple way to combine multiple elements in an array.

squeeze.js

Array.prototype.squeeze = function (n, fn) {
  const from = Object(this).slice();
  const to = [];

  while (n > 0 && from.length)
    to.push(fn.apply(undefined, from.splice(0, n)));

  return to;
}

// Convert [1, 2, 3, 4] to [1 + 2, 3 + 4]
var test = [1, 2, 3, 4];
console.log(JSON.stringify(test.squeeze(2, (a, b) => a + b)));