utils.js

function* range(n) {
    let i = 0;
    while (i++ < n)
        yield i;
}

Array.prototype.sum = function () {
    return this.reduce((a, b) => Number(a) + Number(b));
}

Array.prototype.avg = function () {
    return this.sum() / this.length;
}

for (let i of range(10))
  console.log(i);