Mastering JavaScript Internals#2 The Parser & AST

The Parser: How JavaScript Actually Reads Your Code You type some JavaScript. You hit run. And it works. But between those two moments, something fascinating happens — your code gets read , understood , and transformed before a single instruction executes. That process is called parsing , and it's the very first thing the engine does. Let's walk through it, step by step. 🗺️ The Big Picture: 3 Steps Before Execution When the JavaScript engine receives your code, it does three things in order: Your Code (text) ↓ 1. Tokenizing → breaks code into small labelled pieces ↓ 2. Parsing → builds a tree structure from those pieces ↓ 3. Compiling → turns the tree into something it can run Today we're focusing on steps 1 and 2. Step 3 (compilation & JIT) is Post 3. Step 1: Tokenizing — Breaking Code into Pieces The very first thing the engine does is read your source code character by character and group them into meaningful chunks called tokens . Here's a simple example: const age = 25 ; The engin