Check if the code is running in Jest

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

JavaScriptNode.js & browsers Version 1.0.0 · MIT · .md

function isRunningInJest() { return typeof process !== 'undefined' && process.env.JEST_WORKER_ID !== undefined; }

Usage

// 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.

Know the 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 above is the code the usage lines run. It does not validate its arguments; add checks where it receives untrusted input.

Keep exploring

• Convert Celsius to Fahrenheit — Apply the temperature conversion without rounding the result.
• Convert hex to RGB — Expand #rgb if needed and read the red, green and blue channels.
• Convert URL parameters to object — Read a query string into an object; repeated keys become arrays.