Introduction to AST Query

AST Query is a powerful library for analyzing and transforming code by manipulating its Abstract Syntax Tree (AST). This library provides dozens of useful APIs that make it easier to understand your code’s structure, perform complex transformations, and automate code generation tasks.

Getting Started with AST Query

Before diving into the APIs, ensure you have installed the ast-query library:

  
    npm install ast-query
  

API Examples

1. Parsing Code into AST

  
    const ASTQ = require('ast-query');
    const ast = ASTQ.parse('const x = 10;');
  

2. Finding Nodes

  
    const varDeclarations = ast.find('VariableDeclaration');
    console.log(varDeclarations.length); // Outputs: 1
  

3. Modifying Nodes

  
    const varDeclarations = ast.find('VariableDeclaration');
    varDeclarations.replace('const', 'let');
    console.log(ast.toString());
  

4. Adding Nodes

  
    ast.append('body', 'console.log(x);');
    console.log(ast.toString());
  

5. Removing Nodes

  
    const varDeclarations = ast.find('VariableDeclaration');
    varDeclarations.remove();
    console.log(ast.toString());
  

6. Renaming Variables

  
    ast.find('Identifier[name="x"]').rename('y');
    console.log(ast.toString());
  

7. Generating Code from AST

  
    const modifiedCode = ast.toString();
    console.log(modifiedCode);
  

Example Application Using AST Query APIs

Let’s create an example application that reads JavaScript code, transforms it, and logs the modified code:

  
    const fs = require('fs');
    const ASTQ = require('ast-query');
    
    // Read the original JavaScript code from a file
    const code = fs.readFileSync('example.js', 'utf-8');
    
    // Parse the code into an AST
    const ast = ASTQ.parse(code);
    
    // Find and rename a variable
    ast.find('Identifier[name="oldVarName"]').rename('newVarName');
    
    // Add a new statement
    ast.append('body', 'console.log(newVarName);');
    
    // Remove an outdated statement
    ast.find('VariableDeclaration').remove();
    
    // Generate the modified code
    const modifiedCode = ast.toString();
    
    // Write the modified code back to a file
    fs.writeFileSync('modifiedExample.js', modifiedCode);
    
    console.log('Code transformation complete.');
  

In this example, we read the original JavaScript code from a file, parse it into an AST, rename a variable, add a new logging statement, remove an outdated statement, and finally write the modified code back to a file.

This demonstrates how powerful and flexible AST Query can be for code analysis and transformation tasks.

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