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Parser

Parser

Introduction

The parser converts JavaScript source code into an Abstract Syntax Tree (AST) — a tree representation of the code’s structure.

Parsing Process

flowchart LR
    A["Source: const x = 5;"] --> B["Lexer: Tokenizer"]
    B --> C["Tokens: [const, x, =, 5, ;]"]
    C --> D["Parser: Build AST"]
    D --> E["AST: VariableDeclaration"]

Tokenization (Lexing)

// Source code
const x = 5;

// Tokens produced
// [Keyword: const] [Identifier: x] [Operator: =] [Number: 5] [Punctuator: ;]

AST Structure

Program
 ├── VariableDeclaration (const)
 │   ├── VariableDeclarator
 │   │   ├── Identifier (x)
 │   │   └── NumericLiteral (5)

Eager vs Lazy Parsing

V8 uses two parsing modes:

Eager: Parses functions immediately (slower start)
Lazy: Skips function bodies until called (faster start, used by default)

// Lazily parsed — body skipped until called
function greet() {
  console.log('Hello');
}

// Eagerly parsed with parens
const greet = (function() {
  console.log('Hello');
});

Summary

Parser converts source → AST
Lexer tokenizes, Parser builds tree
V8 uses lazy parsing for faster startup
AST is input for the interpreter

Further Reading