Introduction to AST Query

The AST (Abstract Syntax Tree) Query is a powerful tool used by developers to interact with and manipulate code structures programmatically. This guide will introduce you to the basics of AST Query, followed by a variety of useful API explanations and examples.

Understanding AST Query

AST Query allows developers to parse code into an abstract syntax tree, providing a structured representation of the source code. By using AST Query, you can easily traverse, analyze, and transform code.

Basic AST Query API Examples

Below are some of the useful AST Query APIs with examples:

1. Parsing Code to AST

  const ast = astQuery.parse('const x = 10;');
  console.log(ast);

2. Finding Specific Nodes

  const ast = astQuery.parse('function greet() {console.log("Hello");}');
  const functions = astQuery.find(ast, 'FunctionDeclaration');
  console.log(functions);

3. Modifying AST Nodes

  const ast = astQuery.parse('var y = 5;');
  astQuery.replace(ast, 'Identifier[name="y"]', node => {
    node.name = 'z';
    return node;
  });
  const transformedCode = astQuery.generate(ast);
  console.log(transformedCode); // Outputs: var z = 5;

4. Generating Code from AST

  const ast = astQuery.parse('let sum = a + b;');
  const code = astQuery.generate(ast);
  console.log(code); // Outputs: let sum = a + b;

Application Example

Let’s see an example of an application that uses several of these AST Query APIs.

  // Sample code to transform variable names in source code
  const sourceCode = `
    const a = 10;
    function add(x, y) {
      return x + y;
    }
    const result = add(a, 5);
  `;
  
  // Parse the code to AST
  const ast = astQuery.parse(sourceCode);
  
  // Replace 'a' with 'num'
  astQuery.replace(ast, 'Identifier[name="a"]', node => {
    node.name = 'num';
    return node;
  });

  // Generate transformed code
  const transformedCode = astQuery.generate(ast);
  console.log(transformedCode);
  /* Outputs:
  const num = 10;
  function add(x, y) {
    return x + y;
  }
  const result = add(num, 5);
  */

With the above example, we see how AST Query helps in parsing code, finding nodes, modifying them, and generating back the transformed code.