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snippets/js/s/difference-by.md

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+---
+title: Mapped array difference
+type: snippet
+language: javascript
+tags: [array]
+cover: keyboard
+dateModified: 2020-10-19T18:52:00+03:00
+---
+
+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`.
+- Use `Array.prototype.map()` to apply `fn` to each element in `a`.
+- Use `Array.prototype.filter()` in combination with `fn` on `a` to only keep values not contained in `b`, using `Set.prototype.has()`.
+
+```js
+const differenceBy = (a, b, fn) => {
+  const s = new Set(b.map(fn));
+  return a.map(fn).filter(el => !s.has(el));
+};
+```
+
+```js
+differenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [1]
+differenceBy([{ x: 2 }, { x: 1 }], [{ x: 1 }], v => v.x); // [2]
+```