Mapped array difference

Returns the difference between two arrays, after applying the provided function to each array element of both.

• Create a Set by applying fn to each element in b.
• Use Array.prototype.map() to apply fn to each element in a.
• Use Array.prototype.filter() in combination with fn on a to only keep values not contained in b, using Set.prototype.has().

const differenceBy = (a, b, fn) => {
  const s = new Set(b.map(fn));
  return a.map(fn).filter(el => !s.has(el));
};

differenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [1]
differenceBy([{ x: 2 }, { x: 1 }], [{ x: 1 }], v => v.x); // [2]