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Compiler and Interpreter

Differences between interpreters and compilers in JS engines and why both are used.

Short explanation

Engines often start with an interpreter for quick startup and then JIT-compile frequently-run code for speed. Interpreters execute code directly from bytecode, while compilers produce optimized machine code.

Small snippet

function fib(n) {
  if (n < 2) return n;
  return fib(n - 1) + fib(n - 2);
}
console.log(fib(10));

How JS handles it internally

  • The interpreter runs bytecode quickly while gathering type/profile info.
  • The compiler uses collected data to produce optimized code paths and may deoptimize if assumptions are invalidated.

Diagram

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Related

🔗 Related:

FAQ

Q: What is deoptimization?

A: Deoptimization is when JIT-compiled code assumptions are violated (like unexpected types) and the engine falls back to a safe execution mode.