30 seconds of code

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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));