String Analysis For Dynamic Field Access

In JavaScript, and scripting languages in general, dynamic field access is a commonly used feature. Unfortunately, current static analysis tools either completely ignore dynamic field access or use overly conservative approximations that lead to poor precision and scalability.We present new string domains to reason about dynamic field access in a static analysis tool. A key feature of the domains is that the equal, concatenate and join operations take O(1) time.Experimental evaluation on four common JavaScript libraries, including jQuery and Prototype, shows that traditional string domains are insufficient. For instance, the commonly used constant string domain can only ensure that at most 21% dynamic field accesses are without false positives. In contrast, our string domain H ensures no false positives for up to 90% of all dynamic field accesses.We demonstrate that a dataflow analysis equipped with the H domain gains significant precision resulting in an analysis speedup of more than 1.5x for 7 out of 10 benchmark programs.