get-sum.js

function getSum(a: number, b: number): number {
  if (a === b) {
    return a;
  }
  const [start, end] = [a, b].sort((a, b) => a - b);
  return Array(end - start + 1)
    .fill(null)
    .map((_, idx) => start + idx)
    .reduce((acc, curr) => acc + curr);
}

const foo = getSum(-2, 7);
console.log(foo);