A JavaScript engine is a program that executes JavaScript code by converting it into machine code that the CPU can understand. Modern engines like V8 (Chrome, Node.js), SpiderMonkey (Firefox), and JavaScriptCore (Safari) are highly optimized for speed and efficiency.

1. Main Components

• Call Stack – Executes code within an execution context.

• Heap – An unstructured memory pool for storing objects.

• Parser – Reads source code and generates an Abstract Syntax Tree (AST).

• Interpreter (e.g., Ignition in V8) – Quickly converts AST into bytecode for fast startup.

• Compiler (e.g., TurboFan in V8) – Uses Just-In-Time (JIT) compilation to convert bytecode into optimized machine code.

• Garbage Collector – Frees memory by removing unused objects (often generational and incremental).

2. Execution Flow

1. Parsing – Code is tokenized and transformed into an AST.

2. Baseline Compilation – Bytecode is generated for immediate execution.

3. Optimization – Frequently executed (“hot”) code paths are recompiled into highly optimized machine code.

4. Deoptimization – If assumptions fail, the engine falls back to less optimized code.