Learn how engines like V8 and SpiderMonkey parse, compile, and execute JavaScript code.
A JavaScript engine is a program or interpreter that executes JavaScript code. Every major browser has one: Google Chrome has V8, Mozilla Firefox has SpiderMonkey, and Safari has JavaScriptCore. V8 is also the engine that powers the Node.js runtime. The engine's primary job is to take the JavaScript code developers write and convert it into machine code that a computer's processor can understand and execute. This process is highly optimized for performance. Modern engines use a technique called Just-In-Time (JIT) compilation. Instead of purely interpreting the code line by line (which is slow) or compiling it all before execution (which can have a slow startup), JIT compilation combines the best of both worlds. It starts by interpreting the code, but it also monitors the code as it runs. If it identifies 'hot' segments of code that are executed frequently, it compiles them into optimized machine code for faster execution. The engine's main components include a parser to read the source code and turn it into an Abstract Syntax Tree (AST), an interpreter, and an optimizing compiler. Understanding engines helps you write more performant code by being mindful of how your code will be processed under the hood.