# Check if a value is an async function

Read the internal tag of async function declarations and arrows.

Canonical: https://1loc.dev/function/check-if-a-value-is-an-async-function/
Version: 1.0.0
Runtime: JavaScript · Node.js 22 / modern browsers
Verification: Usage lines executed in Node.js 22 and headless Chromium; no external services
Updated: 2026-10-28

## Implementation

```javascript
function isAsyncFunction(value) { return Object.prototype.toString.call(value) === '[object AsyncFunction]'; }
```

## Usage

```javascript
isAsyncFunction(async () => {}); // → true
isAsyncFunction(() => Promise.resolve()); // → false
isAsyncFunction(function* () {}); // → false
```

Lines marked → were executed in Node.js 22 and in headless Chromium, and each result was compared with the value shown. Lines marked e.g. are illustrative.

## Limits

- A regular function that returns a promise is not detected, as the second line shows, and code transpiled to older JavaScript loses the tag.
- Do not branch on it: await the result, which works for both.

The function does not validate its arguments. Add checks where it receives untrusted input.

## Source and license

Original: https://github.com/phuocng/1loc/blob/14b481cf60dff1620ab1bb99a43b1f68068cdd9b/contents/check-if-a-value-is-an-async-function.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: Documented as a reference with executed usage lines. Original MDX retained as inert source evidence.