Check if the code is running in the browser

Look for the window and document globals.

JavaScriptBrowser · DOM Version 1.0.0 · MIT · .md

function isBrowser() { return typeof window === 'object' && typeof document === 'object'; }

Usage

isBrowser(); // → true

Lines marked → were executed in headless Chromium against the markup above, and each result was compared with the value shown. Lines marked e.g. are illustrative.

Know the limits

• Returns false in web and service workers, which have no document, and in Node.js, as the unit tests confirm.
• DOM emulators such as jsdom define both globals, so tests running under them see true.

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.