The validation of formal models is a challenge for formal methods. We propose JeB, a framework which generates and executes simulations of Event-B models, even highly nondeterministic ones. JeB allows users to safely insert pieces of code to supply deterministic computations where the automatic translation fails. We present how JeB translates Event-B model into JavaScript. We define Fidelity as the formal notion which captures the idea of the correctness of a simulation. We define it through proof-obligations.