We present the first implementation of Thread-Level Speculation in combination with Just-in-time compilation. The implementation is done in Google’s V8, a well-known JavaScript engine, and evaluated on 15 popular web application executing on 2, 4, and 8 core computers. Our results show an average speedup of 2.9 on 4 cores, without any JavaScript code modifications. Further, we have found that the Just-in-time compilation time is significant, and that most functions are lazily compiled (approximately 80%) and that V8 contains features that are advantageous in Thread-Level Speculation.