differenceBy

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, then use Array.prototype.map() to apply fn to each element in a, then Array.prototype.filter()

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]