---
title: Unique values in array based on function
tags: array
cover: sunrise-over-mountains
firstSeen: 2018-07-18T20:49:07+03:00
lastUpdated: 2021-10-13T19:29:39+02:00
---

Finds all unique values of an array, based on a provided comparator function.

- Use `Array.prototype.reduce()` and `Array.prototype.some()` to create an array containing only the first unique occurrence of each value, based on the comparator function, `fn`.
- The comparator function takes two arguments: the values of the two elements being compared.

```js
const uniqueElementsBy = (arr, fn) =>
  arr.reduce((acc, v) => {
    if (!acc.some(x => fn(v, x))) acc.push(v);
    return acc;
  }, []);
```

```js
uniqueElementsBy(
  [
    { id: 0, value: 'a' },
    { id: 1, value: 'b' },
    { id: 2, value: 'c' },
    { id: 1, value: 'd' },
    { id: 0, value: 'e' }
  ],
  (a, b) => a.id == b.id
); // [ { id: 0, value: 'a' }, { id: 1, value: 'b' }, { id: 2, value: 'c' } ]
```