Creating JavaScript Using JavaScript
Ever wondered how JavaScript itself understands and runs code? What if we told you that you can create JavaScript using JavaScript? In this article, we'll guide you through building a simple JavaScript interpreter, starting with the easy way (eval) and then diving into the fundamentals like lexers, parsers, and evaluators.
🧪 The Easiest Way: Using eval()
The eval() function in JavaScript takes a string and executes it as JavaScript code. It's essentially a built-in interpreter.
Why Not Just Use eval()?
- Security Risk: It runs arbitrary code, which could be dangerous.
- Debugging Difficulty: Hard to trace errors.
- No Control: You can't customize how the code is interpreted.
So let's move on to the better way: building your own interpreter.
🛠️ Step-by-Step: Build a Basic JavaScript Interpreter
We'll now create a simple JavaScript-like interpreter that supports basic math and variables. To do this, we'll break it down into parts:
- Lexer - Break code into tokens
- Parser: Convert tokens into an Abstract Syntax Tree (AST)
- Evaluator: Executes the AST and returns the result.
1️⃣ Lexer (Tokenizer)
A lexer splits your code string into tokens — like numbers, operators, and identifiers.
Example Input:
let x = 5 + 3;Example Output:
Basic Lexer Implementation
Here's a simple lexer that can handle basic variable assignments and arithmetic operations:
2️⃣ Parser (Build AST)
We convert tokens into a tree structure (AST) to make it easier to evaluate.
Example AST Output for let x = 5 + 3;:
Basic Parser Code:
Here's a simple parser that can handle variable declarations and basic arithmetic:
3️⃣ Evaluator
Now we run the AST like a real JavaScript interpreter.
Basic Evaluator Code:
✅ Final Example
🧠 Conclusion
You just created a tiny JavaScript interpreter in JavaScript!
- ✅ eval is easy but unsafe
- ✅ Writing your own interpreter gives you full control
- 🛠️ The core steps are: Tokenize → Parse → Evaluate
This is just the beginning. You can expand this to support:
- More operators (like -, *, /)
- Control flow (if statements, loops)
- Functions and scopes
- Error handling