Almost all operations in JavaScript can be effectful and dynamic. For example, the plus operator may simply add two numbers, or it might invoke function calls at runtime. To run JavaScript programs efficiently, JavaScriptCore heavily speculates on what these operations do at runtime. JavaScriptCore has two compilers built on its DFG IR, which uses speculation as a foundational construct in its IR design. I’ll discuss how DFG IR supports speculation and OSR exit as a foundational building block, how we leverage speculation to enhance static analysis, and how we design IR operations to improve optimization capabilities.