30-seconds-of-code/snippets/intersection-by.md

title, tags, cover, firstSeen, lastUpdated

title	tags	cover	firstSeen	lastUpdated
Mapped array intersection	array	cobbled-street	2018-01-24T12:53:18+02:00	2020-10-20T23:02:01+03:00

Returns the 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.
• 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(b.map(fn));
  return [...new Set(a)].filter(x => s.has(fn(x)));
};

intersectionBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [2.1]
intersectionBy(
  [{ title: 'Apple' }, { title: 'Orange' }],
  [{ title: 'Orange' }, { title: 'Melon' }],
  x => x.title
); // [{ title: 'Orange' }]