Deep dive into V8 JS Engine

Parsing Stage:

• When JavaScript code is given to the V8 engine, the first step is lexical analysis, where the code is broken into tokens.

• Then, it moves to syntax analysis, where the tokens are converted into an Abstract Syntax Tree (AST).

• 🚀 Tech Fact: "No AST, No Execution! A SyntaxError means your code didn’t even make it past the parsing stage

Abstract Syntax Tree (AST) representation for the JavaScript code:

var a = "priyanshu";

{
  "type": "VariableDeclaration",
  "declarations": [
    {
      "type": "VariableDeclarator",
      "id": {
        "type": "Identifier",
        "name": "a"
      },
      "init": {
        "type": "Literal",
        "value": "priyanshu",
        "raw": "\"priyanshu\""
      }
    }
  ],
  "kind": "var"
}

• Execution Stage:

  • The AST is then passed to an interpreter for execution.

• Compiler vs Interpreter:

  • Interpreter: Executes code line by line, making the initial execution fast but overall slower.

  • Compiler: Translates the entire code into machine code before execution, which makes the initial process heavy but execution fast.

• Is JavaScript a Compiler or Interpreter?

  • JavaScript uses Just-In-Time (JIT) Compilation, meaning it compiles code at runtime, combining the benefits of both interpreters and compilers.

1. Execution Begins (Interpreter - Ignition)

• After parsing, the Abstract Syntax Tree (AST) is passed to Ignition, the interpreter inside V8.

• Ignition converts the AST into bytecode and begins executing it.

2. Optimization (Compiler - TurboFan)

• While executing, Ignition detects frequently used functions or code patterns that can be optimized.

• These "hot" functions are passed to TurboFan, the JIT (Just-In-Time) compiler.

• TurboFan compiles the bytecode into highly optimized machine code.

• The optimized machine code is then executed instead of the slower bytecode.

3. Deoptimization (When Needed)

• If the assumptions made by TurboFan for optimization turn out to be incorrect (e.g., variable types change), the optimized machine code is discarded.

• The execution falls back to the bytecode from Ignition, ensuring correctness.