Implement intersectionBy, differenceBy, unionBy

Implement intersectionBy, differenceBy, unionBy

The three classic set operations, generalized to work on arrays of objects by comparing a key derived from each item via keyFn (not the objects themselves, two different objects can represent "the same" item).

• intersectionBy(arr1, arr2, keyFn), items in arr1 whose key also appears in arr2.
• differenceBy(arr1, arr2, keyFn), items in arr1 whose key does not appear in arr2.
• unionBy(arr1, arr2, keyFn), all items from both, deduplicated by key (first occurrence wins), in order.

const a = [{ id: 1 }, { id: 2 }];
const b = [{ id: 2 }, { id: 3 }];
intersectionBy(a, b, (x) => x.id); // [{ id: 2 }]
differenceBy(a, b, (x) => x.id);   // [{ id: 1 }]
unionBy(a, b, (x) => x.id);        // [{ id: 1 }, { id: 2 }, { id: 3 }]