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Getting Started with Tree-sitter: Syntax Trees and Express API Parsing

Tree-sitter is a powerful parser generator tool that enables you to build efficient and incremental parsers for programming languages. Whether you're building code analyzers, linters, or even editors, Tree-sitter allows you to dive deep into code and work with its syntax tree, providing structured insights into the language's grammar. In this post, we’ll go over the basics of using Tree-sitter to parse JavaScript code and how you can use it to parse Express API routes, including middleware. By the end of this post, you’ll be able to build your own custom code analysis tools with ease. What Can Tree-sitter Be Used For? Tree-sitter opens up many possibilities for analyzing and interacting with code in a structured way. Here are some common use cases for Tree-sitter: 1. Syntax Highlighting Tree-sitter is widely used in code editors like Visual Studio Code and Atom for syntax highlighting. By parsing the code into a syntax tree, Tree-sitter can help identify language constructs (like keywords, variables, and functions) and apply distinct colors, improving the readability and developer experience. 2. Code Navigation and Refactoring Tree-sitter enables tools to understand the structure of the code, making navigation and refactoring much easier. With the syntax tree, you can find function definitions, track variable usage, and even perform automated refactoring like renaming variables or functions across a codebase with high accuracy. 3. Static Analysis and Linting Tree-sitter helps in writing custom linting tools. These tools can analyze code without executing it, detecting common issues or enforcing style guidelines. The syntax trees produced by Tree-sitter allow for deeper analysis than traditional regex-based linters, making it possible to check for complex patterns like unused variables or unreachable code. 4. Code Completion and Autocompletion Tree-sitter can be used to power autocompletion in IDEs and code editors. By analyzing the syntax tree of the code, it can predict the next valid tokens or function signatures based on the current context, enhancing the coding experience. 5. Code Formatting and Formatting Checks Tree-sitter makes it easier to write tools that automatically format code according to style rules. By parsing the code into a syntax tree, you can rebuild it in a consistent format, applying indentation rules and formatting guidelines programmatically. 6. Custom Code Linters and Analyzers You can use Tree-sitter to write custom analysis tools specific to your codebase or programming language. For example, you can write a custom linter that checks if all function declarations have proper documentation or if code is using deprecated API methods. 7. Documentation Generation Tree-sitter can be used to automate documentation generation by analyzing function signatures, comments, and code structure. This helps generate up-to-date API docs directly from the source code, saving time and ensuring consistency. 8. Building Custom IDE Features If you’re building your own integrated development environment (IDE) or plugin, Tree-sitter can help you add powerful features like context-aware auto-completion, error detection, and inline documentation. By leveraging the syntax trees, your IDE can provide smarter code suggestions and real-time error detection. 9. Parsing Non-Programming Languages While Tree-sitter is often used for programming languages, it can also be adapted to parse other structured text formats, like JSON, Markdown, or even domain-specific languages (DSLs). This makes it versatile for building tools that need to understand custom formats beyond typical programming languages. Basic Setup What is Tree-sitter? Tree-sitter is a parsing library designed to efficiently generate concrete syntax trees from source code in various programming languages. These trees give you detailed information about the structure of the code, enabling more powerful analysis and manipulation. You can use Tree-sitter for a variety of tasks, such as: Building syntax-aware editors Analyzing code Implementing linters or formatters Extracting information from code Installing Tree-sitter Before we start using Tree-sitter, let’s install the necessary packages: First, make sure you have node.js installed. Then, install Tree-sitter using npm: npm install tree-sitter tree-sitter-javascript The tree-sitter-javascript package provides a parser for JavaScript code. Parsing Basic JavaScript Code Now, let’s create a basic example where we’ll parse a simple JavaScript function using Tree-sitter. const Parser = require("tree-sitter"); const JavaScript = require("tree-sitter-javascript"); // Create a parser const parser = new Parser(); parser.setLanguage(JavaScript); // Sample code to parse const code = ` function greet(name) { return

Apr 15, 2025 - 05:27
 0
Getting Started with Tree-sitter: Syntax Trees and Express API Parsing

Tree-sitter is a powerful parser generator tool that enables you to build efficient and incremental parsers for programming languages.

Whether you're building code analyzers, linters, or even editors, Tree-sitter allows you to dive deep into code and work with its syntax tree, providing structured insights into the language's grammar.

In this post, we’ll go over the basics of using Tree-sitter to parse JavaScript code and how you can use it to parse Express API routes, including middleware.

By the end of this post, you’ll be able to build your own custom code analysis tools with ease.

What Can Tree-sitter Be Used For?

Tree-sitter opens up many possibilities for analyzing and interacting with code in a structured way.
Here are some common use cases for Tree-sitter:

1. Syntax Highlighting

Tree-sitter is widely used in code editors like Visual Studio Code and Atom for syntax highlighting.
By parsing the code into a syntax tree, Tree-sitter can help identify language constructs (like keywords, variables, and functions) and apply distinct colors, improving the readability and developer experience.

2. Code Navigation and Refactoring

Tree-sitter enables tools to understand the structure of the code, making navigation and refactoring much easier.
With the syntax tree, you can find function definitions, track variable usage, and even perform automated refactoring like renaming variables or functions across a codebase with high accuracy.

3. Static Analysis and Linting

Tree-sitter helps in writing custom linting tools. These tools can analyze code without executing it, detecting common issues or enforcing style guidelines.
The syntax trees produced by Tree-sitter allow for deeper analysis than traditional regex-based linters, making it possible to check for complex patterns like unused variables or unreachable code.

4. Code Completion and Autocompletion

Tree-sitter can be used to power autocompletion in IDEs and code editors.
By analyzing the syntax tree of the code, it can predict the next valid tokens or function signatures based on the current context, enhancing the coding experience.

5. Code Formatting and Formatting Checks

Tree-sitter makes it easier to write tools that automatically format code according to style rules.
By parsing the code into a syntax tree, you can rebuild it in a consistent format, applying indentation rules and formatting guidelines programmatically.

6. Custom Code Linters and Analyzers

You can use Tree-sitter to write custom analysis tools specific to your codebase or programming language.
For example, you can write a custom linter that checks if all function declarations have proper documentation or if code is using deprecated API methods.

7. Documentation Generation

Tree-sitter can be used to automate documentation generation by analyzing function signatures, comments, and code structure. This helps generate up-to-date API docs directly from the source code, saving time and ensuring consistency.

8. Building Custom IDE Features

If you’re building your own integrated development environment (IDE) or plugin, Tree-sitter can help you add powerful features like context-aware auto-completion, error detection, and inline documentation. By leveraging the syntax trees, your IDE can provide smarter code suggestions and real-time error detection.

9. Parsing Non-Programming Languages

While Tree-sitter is often used for programming languages, it can also be adapted to parse other structured text formats, like JSON, Markdown, or even domain-specific languages (DSLs). This makes it versatile for building tools that need to understand custom formats beyond typical programming languages.

Basic Setup

What is Tree-sitter?

Tree-sitter is a parsing library designed to efficiently generate concrete syntax trees from source code in various programming languages.

These trees give you detailed information about the structure of the code, enabling more powerful analysis and manipulation.

You can use Tree-sitter for a variety of tasks, such as:

  • Building syntax-aware editors
  • Analyzing code
  • Implementing linters or formatters
  • Extracting information from code

Installing Tree-sitter

Before we start using Tree-sitter, let’s install the necessary packages:

  1. First, make sure you have node.js installed.
  2. Then, install Tree-sitter using npm: 
npm install tree-sitter tree-sitter-javascript

The tree-sitter-javascript package provides a parser for JavaScript code.

Parsing Basic JavaScript Code

Now, let’s create a basic example where we’ll parse a simple JavaScript function using Tree-sitter.

const Parser = require("tree-sitter");
const JavaScript = require("tree-sitter-javascript");

// Create a parser
const parser = new Parser();
parser.setLanguage(JavaScript);

// Sample code to parse
const code = `
function greet(name) {
  return 'Hello, ' + name + '!';
}
`;

// Parse it
const tree = parser.parse(code);

// Print the syntax tree
console.log(tree.rootNode.toString());

Explanation:

  1. We create a Parser object and set its language to JavaScript using setLanguage().
  2. We define a simple JavaScript function, greet, that takes one argument name and returns a greeting message.
  3. We parse the code and get a syntax tree object. The rootNode of the tree represents the root of the abstract syntax tree (AST).
  4. Finally, we print out the tree using tree.rootNode.toString(), which will give us a string representation of the AST.

Sample Output: 

(program
  (function_declaration
    name: (identifier)
    parameters: (formal_parameters (identifier))
    body: (statement_block
      (return_statement
        (binary_expression
          left: (binary_expression
            left: (string (string_fragment))
            right: (identifier))
          right: (string (string_fragment)))))))

This output shows the tree structure of the greet function.

Each node in the tree represents a syntactic construct, and you can navigate this structure to analyze specific parts of the code.

Parsing Express API Routes with Middleware

Now that we've covered the basics of parsing JavaScript code, let’s move on to a more advanced example: parsing Express.js API routes along with their middleware functions.

In this example, we’ll take a small Express API that uses middleware and extract relevant information such as the HTTP method (GET, POST, etc.), the API route, and the middleware used.

Example Express API Code: 

const express = require('express');
const app = express();
const router = express.Router();

const authenticateMiddleware = (req, res, next) => { ... };
const logRequestMiddleware = (req, res, next) => { next(); };

app.use('/api', authenticateMiddleware);
app.get('/api/users', authenticateMiddleware, (req, res) => {
  res.send('User list');
});

router.post('/api/login', logRequestMiddleware, loginHandler);

Parsing the Express API

Now, let’s write a parser using Tree-sitter to analyze the Express API code and extract the routes and middleware.

const Parser = require("tree-sitter");
const JavaScript = require("tree-sitter-javascript");
const { Query } = require("tree-sitter");

const parser = new Parser();
parser.setLanguage(JavaScript);

// Sample Express code with middleware
const code = `
const express = require('express');

// Example Express API Code ...
`;

const tree = parser.parse(code);
const root = tree.rootNode;
console.log(tree.rootNode.toString()); //
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