Returns a list of elements that exist in both arrays, after applying the provided function to each array element of both.

Create a Set by applying fn to all elements in b, then use Array.prototype.filter() on a to only keep elements, which produce values contained in b when fn is applied to them.


const intersectionBy = (a, b, fn) => {
  const s = new Set(;
  return [ Set(a)].filter(x => s.has(fn(x)));


intersectionBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [2.1]