# Find the index of the maximum item of an array

Return the index of the first largest number.

Canonical: https://1loc.dev/array/find-the-index-of-the-maximum-item-of-an-array/
Version: 1.0.0
Runtime: JavaScript · Node.js 22 / modern browsers
Verification: Local fixtures; no external services
Updated: 2026-10-28

## Implementation

```javascript
function indexOfMax(values) { return values.reduce((best, value, i) => (value > values[best] ? i : best), 0); }
```

## Input

```json
[
  1,
  3,
  9,
  7,
  9
]
```

## Expected output

```json
2
```

## Edge case

Input:
```json
[]
```
Output:
```json
0
```

## Limits

- The first maximum wins on ties. An empty array returns 0, as in the original — check the length first.

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/find-the-index-of-the-maximum-item-of-an-array.mdx
Author: Nguyen Huu Phuoc and original contributors
Contributors: See original repository history
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.