AOT vs JIT

AOT does not necessarily mean compiling a program to native machine code. The speaker defines AOT and JIT by when compilation and related optimizations happen, regardless of the source or output language. Examples include Angular compiling templates and TypeScript to JavaScript, and Clojure compiling to JVM bytecode.

Java compiles code to bytecode ahead of time, while a JVM may interpret that bytecode or JIT compile it to machine code. Hermes follows a similar model, so adding a JIT is technically possible. The team experimented with one but removed it because its value did not justify the overhead.

With Hermes, JavaScript source is precompiled to bytecode during the React Native app build, with optimizations such as inlining, code elimination, and loop unrolling. At runtime, the app executes that bytecode, so the engine does not need to parse and compile the JavaScript then. The speaker says enabling Hermes is generally a simple switch, and React Native 0.64 made support easier across platforms.

AOT and JIT name when work happens; they do not specify whether the output is native code or what the work does. The speaker points out that libraries with runtimes can do work just in time, while libraries that transpile everything to CSS can do it ahead of time.