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snippets/js/s/intersection-by.md

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+---
+title: Mapped array intersection
+type: snippet
+language: javascript
+tags: [array]
+cover: cobbled-street
+dateModified: 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.
+
+```js
+const intersectionBy = (a, b, fn) => {
+  const s = new Set(b.map(fn));
+  return [...new Set(a)].filter(x => s.has(fn(x)));
+};
+```
+
+```js
+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' }]
+```