Add intersectionBy, intersectionWith

snippets/intersectionBy.md

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+### intersectionBy
+
+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.filter()` on `a` to only keep elements, which produce values contained in `b` when `fn` is applied to them.
+
+```js
+const intersectionBy = (a, b, fn) => {
+  const s = new Set(b.map(x => fn(x)));
+  return a.filter(x => s.has(fn(x)));
+};
+```
+
+```js
+intersectionBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [2.1]
+```