Tracing just-in-time compilers (TJITs) determine frequently executed traces (hot paths and loops) in running programs and focus their optimization effort by emitting optimized machine code specialized to these traces. Prior work has established this strategy to be especially beneficial for dynamic languages such as JavaScript, where the TJIT interfaces with the interpreter and produces machine code from the JavaScript trace.

Web sites and web browsers have recently evolved into platforms on top of which entire applications are delivered dynamically, mostly as JavaScript source code. This delivery format has sparked extremely enthusiastic efforts to customize both individual web sites and entire browsers in ways the original authors never expected or accommodated. Such customizations take the form of yet more script dynamically injected into the application, and the current idioms to do so exploit arcane JavaScript features and are extremely brittle. In this work, we accept the popularity of extensions and seek better linguistic mechanisms to support them.