An Invisible JavaScript Device Challenge is a security measure that forces a browser to execute a javascript challenge before allowing access to the requested page. It operates without requiring any user interaction, leveraging techniques like proof of work, device fingerprinting, and tests to determine whether the browser is being driven by automation or spoofing some aspect of its behaviour.
Typically, on passing the device challenge, a cookie is set in the browser to allow further browsing without further challenges.
What are the advantages of an Invisible Javascript Challenge?
Since the challenge doesn't require any human interaction they are seen as 'frictionless' when compared to CAPTCHAS. They don't frustrate users by forcing them to solve annoying puzzles. However, they do require that Javascript be enabled in the browser, creating accessibility issues with some clients.