JavaScript Challenge is a type of Web Challenge that is used in DDoS mitigation to filter out attackers from legitimate clients.
The challenge is to send every client, attacker and legitimate user a JavaScript code that includes some kind of challenge. Virtually any legitimate browser support has a JavaScript stack and will easily understand and pass the challenge transparently (without the user’s notice), while DDoS bots typically are not equipped with JavaScript stack and therefore cannot pass the challenge.
Related entries: Cookie Validation, Web Challenges, Web Challenge Spectrum