Mapped array difference

OSCC · JavaScript, Array · Oct 19, 2020

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]

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