Type Inference is Brian Hackett's type inference engine for JavaScript. It is a whole-program, hybrid static and dynamic analysis that attempts to find the set of possible types for stack slots, arguments, and local variables.

Some information can only be deduced at run-time, for example, the effects of an eval or integer overflow. If assumptions about type information are broken, the analysis is re-run on anything depending on that information. This can cause methods to be recompiled, even replacing currently running methods on the stack.