isBrowser javascript Code Snippet in 2025

Determines if the current runtime environment is a browser so that front-end modules can run on the server (Node) without throwing errors.

• Use Array.prototype.includes() on the typeof values of both window and document (globals usually only available in a browser environment unless they were explicitly defined), which will return true if one of them is undefined.
• typeof allows globals to be checked for existence without throwing a ReferenceError.
• If both of them are not undefined, then the current environment is assumed to be a browser.

const isBrowser = () => ![typeof window, typeof document].includes('undefined');

isBrowser(); // true (browser)
isBrowser(); // false (Node)