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+---
+title: allUniqueBy
+tags: array,intermediate
+---
+
+Checks if all elements in an array are unique, based on the provided mapping function.
+
+- Use `Array.prototype.map()` to apply `fn` to all elements in `arr`.
+- Create a new `Set` from the mapped values to keep only unique occurences.
+- Use `Array.prototype.length` and `Set.prototype.size` to compare the length of the unique mapped values to the original array.
+
+```js
+const allUniqueBy = (arr, fn) => arr.length === new Set(arr.map(fn)).size;
+```
+
+```js
+allUniqueBy([1.2, 2.4, 2.9], Math.round); // true
+allUniqueBy([1.2, 2.3, 2.4], Math.round); // false
+```