[JS] Pair sums

pairSums.js

// Given a list of integers determine the number of different pairs of elements within it which sum to k.

function numberOfWays(arr, k) {
  let counter = 0;
  const indexesByModK = {};
  arr.forEach((num, j) => {
    const modK = num % k;
    const modKComplement = (k - modK) % k;
    if(Object.keys(indexesByModK).includes(String(modKComplement))) {
      counter += indexesByModK[modKComplement].length;
    }  
    if(indexesByModK[modK]) indexesByModK[modK].push(j);
    else indexesByModK[modK] = [j];
  });
  return counter;
}