# Intersperse element between elements

Insert a separator between every pair of items.

Canonical: https://1loc.dev/array/intersperse-element-between-elements/
Version: 1.0.0
Runtime: JavaScript · Node.js 22 / modern browsers
Verification: Local fixtures; no external services
Updated: 2026-10-28

## Implementation

```javascript
function intersperse({ values, separator }) { return values.flatMap((value, i) => (i === 0 ? [value] : [separator, value])); }
```

## Input

```json
{
  "values": [
    "ana",
    "rui",
    "lia"
  ],
  "separator": ","
}
```

## Expected output

```json
[
  "ana",
  ",",
  "rui",
  ",",
  "lia"
]
```

## Edge case

Input:
```json
{
  "values": [],
  "separator": ","
}
```
Output:
```json
[]
```

## Limits

- The original built Array(2n − 1), which throws for an empty array; flatMap returns [].

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/intersperse-element-between-elements.mdx
Author: Nguyen Huu Phuoc and original contributors
Contributors: iamandrewluca
Source commit: 14b481cf60dff1620ab1bb99a43b1f68068cdd9b
Copyright (c) 2020 phuoc-ng
License: MIT — https://1loc.dev/licenses/
Maintainer: Oveyon
Modifications: Uses flatMap; the original threw for an empty array. Original MDX retained as inert source evidence.