# Get unique objects from an array by key

Keep one record per key value, the last one seen.

Canonical: https://1loc.dev/object/get-unique-arr-obj/
Version: 1.0.0
Runtime: JavaScript · Node.js 22 / modern browsers
Verification: Local fixtures; no external services
Updated: 2026-10-28

## Implementation

```javascript
function uniqueBy({ objects, key }) { return [...new Map(objects.map(item => [item[key], item])).values()]; }
```

## Input

```json
{
  "objects": [
    {
      "recipient": "ana@example.com",
      "status": "deferred"
    },
    {
      "recipient": "ana@example.com",
      "status": "delivered"
    },
    {
      "recipient": "rui@example.com",
      "status": "delivered"
    }
  ],
  "key": "recipient"
}
```

## Expected output

```json
[
  {
    "recipient": "ana@example.com",
    "status": "delivered"
  },
  {
    "recipient": "rui@example.com",
    "status": "delivered"
  }
]
```

## Edge case

Input:
```json
{
  "objects": [],
  "key": "id"
}
```
Output:
```json
[]
```

## Limits

- The last record per key wins, in the position of the first one — handy for keeping the latest state per recipient.

The lab validates types and size before calling this function. Add equivalent validation at your own trust boundary.

## Source and license

Original: https://github.com/phuocng/1loc/blob/14b481cf60dff1620ab1bb99a43b1f68068cdd9b/contents/get-unique-arr-obj.mdx
Author: Nguyen Huu Phuoc and original contributors
Contributors: gpbuavv
Source commit: 14b481cf60dff1620ab1bb99a43b1f68068cdd9b
Copyright (c) 2020 phuoc-ng
License: MIT — https://1loc.dev/licenses/
Maintainer: Oveyon
Modifications: Curated implementation; explicit input contract, edge case and limits. Original MDX retained as inert source evidence.