# Check if a value is a generator function

Read the internal tag of function* and async function* declarations.

Canonical: https://1loc.dev/function/check-if-a-value-is-a-generator-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 isGeneratorFunction(value) { return ['[object GeneratorFunction]', '[object AsyncGeneratorFunction]'].includes(Object.prototype.toString.call(value)); }
```

## Usage

```javascript
isGeneratorFunction(function* () {}); // → true
isGeneratorFunction(async function* () {}); // → true
isGeneratorFunction(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

- The original missed async generator functions, as the second line shows. Symbol.toStringTag can spoof the tag.
- A plain function that returns an iterator is not detected; to consume any iterable, check the value it returns for Symbol.iterator.

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-a-generator-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: Also detects async generator functions. Documented as a reference with executed usage lines. Original MDX retained as inert source evidence.