# Check if the code is running in Jest

Look for the JEST_WORKER_ID variable that Jest sets in its workers.

Canonical: https://1loc.dev/misc/check-if-the-code-is-running-in-jest/
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 isRunningInJest() { return typeof process !== 'undefined' && process.env.JEST_WORKER_ID !== undefined; }
```

## Usage

```javascript
// These checks run under node --test and Chromium, not Jest:
isRunningInJest(); // → 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

- Other runners, such as Vitest or node --test, are not detected.
- Prefer configuring behavior explicitly, with a variable your code reads, over detecting the test runner.

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-the-code-is-running-in-jest.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.