Remove entry from array using another array

Question

Not sure how do to this, so any help is greatly appreciated

Say I have :

const array1 = [1, 1, 2, 3, 4];
const array2 = [1, 2];

Desired

Answer 1

This will run reasonably well. I think its linear time instead of N*N

function diffOnlyOncePerElementInstance(a1, a2) {
  const max = Math.max(a1.length, a2.length);
  const map = {};

  for (let i = 0; i < max; i++) {
    const valueA = a1[i];
    const valueB = a2[i];
    if (i < a1.length) {
      if (!Number.isInteger(map[valueA])) {
        map[valueA] = 0;
      }
      map[valueA]++;
    }
    if (i < a2.length) {
      if (!Number.isInteger(map[valueB])) {
        map[valueB] = 0;
      }
      map[valueB]--
    }
  }

  return Object.keys(map)
    .map(key => new Array(Math.abs(map[key])).fill(key)) // regenerate remaining count
    .reduce((a,b) => a.concat(b), []); // flatten
}