Understanding AST Parsers in TypeScript: Real Use Case from a Math Equation Editor

When you're working with TypeScript or any language that compiles or transforms code, you’ll eventually hear about something called an AST—an Abstract Syntax Tree. It sounds like something out of a compiler theory textbook (and it is), but knowing how it works can level up your development skills, especially if you’re working with tools like linters, code formatters, compilers, or even AI-based code analyzers.

What is an AST?

An Abstract Syntax Tree (AST) is a structured, hierarchical representation of your source code. Think of it as a tree where each node represents a piece of your code, like a variable declaration, function call, or conditional statement.

The key point here is: ASTs don't care about formatting, comments, or syntax sugar, they focus on the logic and structure of your code. This abstraction is what makes it possible for machines (like linters and compilers) to understand your code.

Why Should You Care?

Most developers don’t realize they interact with ASTs all the time - indirectly. Every time your code gets:

• Linted by ESLint,

• Transpiled by the TypeScript compiler,

• Formatted by Prettier,

• Auto-suggested by your IDE like VS Code,

• Transformed by tools like Babel,

an AST is involved. Understanding ASTs helps you:

• Create smarter code tools.

• Understand what's going on behind the scenes.

• Write plugins or codemods.

• Manipulate code safely and programmatically.

If you're into writing clean, scalable, and automated code—ASTs are your secret weapon.

How Does an AST Parser Work?

Let’s break it down into steps that most parsers follow:

1. Lexical Analysis (Tokenization)

   • The parser reads the raw source code and breaks it into tokens—basic units like keywords (const, function), identifiers (x, myFunc), operators (=, +), etc.

2. Parsing (Syntactic Analysis)

   • These tokens are then fed into a parser that follows the language grammar and produces a structured representation—our AST. This step checks for syntax correctness too.

3. AST Generation

   • The final AST is a JSON-like object tree where each node has a type and children. This data structure becomes the foundation for static analysis, transformations, and more.

A Quick Example

Let’s say we write this simple line in TypeScript:

const x = 10;

The AST output might look like this:

{
  "type": "VariableDeclaration",
  "kind": "const",
  "declarations": [
    {
      "type": "VariableDeclarator",
      "id": { "type": "Identifier", "name": "x" },
      "init": { "type": "Literal", "value": 10 }
    }
  ]
}

This structure tells tools exactly what the code does, without needing to interpret it as raw text.

Real-World Use Cases in TypeScript

TypeScript’s compiler API gives you access to a deeply detailed AST, which includes types. Here’s what you can do with it:

• Create custom ESLint rules that enforce specific coding practices.

• Write codemods to automatically refactor your codebase (e.g., rename all functions that start with handle to on).

• Build code analyzers that can find type mismatches, deprecated patterns, or unsafe logic.

• Generate new code (e.g., automatically generate React props interfaces from backend schemas).

• Enable intelligent code completion (as seen in VS Code, which relies heavily on the TypeScript AST).

Why is This Important to Know?

Most people never need to work with ASTs directly unless they’re building tooling, but here’s why you might want to learn:

• You can debug better: If you understand what your compiler is seeing, you can solve complex bugs faster.

• You’ll write smarter tools: Want to automate migrations or transform codebases? ASTs make it possible.

• You’ll grow as a developer: This knowledge puts you in a different league—where you don’t just write code, you shape how code behaves.

Real-Time Example: Building a Math Equation Editor with AST

I was once working on a web-based math equation editor where users could type mathematical expressions like 3x + 4 = 10 or integrate(x^2) and get real-time evaluation and symbolic transformations. The tool needed to not only evaluate arithmetic but also understand more complex algebraic and calculus-based expressions.

Initially, I tried using simple regex and expression trees, but they quickly fell apart when users wrote more complex equations, especially with nested expressions or invalid syntax. I couldn’t differentiate between syntax errors, variables, or functions, and building a safe evaluator was becoming risky.

So I turned to AST parsing. I built a custom parser that would convert the equation into an AST representation:

• I could walk the tree and identify the types of nodes—numbers, operators, variables, functions.

• I used the AST to validate the expression structure before evaluation.

• I built transformation logic like simplifying expressions or converting x^2 + 2x + 1 into (x + 1)^2 by analyzing the AST pattern.

• I integrated evaluation logic that could safely traverse the AST and compute the final result.

The performance and accuracy improved drastically. The equation editor became smarter, handled invalid input gracefully, and even provided real-time syntax feedback—just like a code linter.

This experience taught me that ASTs aren’t just for programming languages—they’re powerful for any domain where structure matters, including mathematics.

Tools You Should Know

• TypeScript Compiler API: For full AST access and type resolution.

• ts-morph: A wrapper around the TypeScript Compiler API to make it easier to manipulate code.

• babel-parser: Widely used in the JavaScript ecosystem to parse modern JavaScript and TypeScript.

• AST Explorer: Online tool that visualizes ASTs of code snippets—great for learning and debugging.

Final Thoughts

Abstract Syntax Trees (ASTs) form the foundation of modern development tooling, especially in statically typed languages like TypeScript. They allow for a structured and reliable understanding of code beyond simple text manipulation, enabling deep integrations across compilers, editors, linters, formatters, and code analyzers.

For developers building robust systems, understanding how ASTs work unlocks the ability to perform advanced transformations, automate refactoring, ensure safe code evaluation, and build reliable developer tools. The TypeScript compiler's exposure of its AST structure, along with libraries like ts-morph, empowers developers to write scalable and intelligent tooling.

Whether you're parsing code for validation, generating documentation, refactoring large codebases, or building domain-specific interpreters—ASTs provide a consistent and powerful abstraction layer. Investing time in understanding and working with ASTs can lead to more maintainable and automated development workflows.

Mastering ASTs isn’t just about writing better code—it’s about building better systems that understand, manipulate, and optimize code at scale.

- Jagadhiswaran Devaraj

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