Node.js

To detect if your script is running in Node.js:

typeof process === 'object' && process.title === 'node';

jsdom

To detect if your script is running in jsdom:

typeof window === 'object' && window.name === 'nodejs';

Or check the user agent:

typeof navigator === 'object' && navigator.userAgent === 'node.js';

See the issue that inspired this post.