Introduction to ast-query

The ast-query library is a powerful tool for developers to navigate and manipulate Abstract Syntax Trees (AST) with ease. Whether you are refactoring large codebases, generating code dynamically, or performing complex source code transformations, ast-query provides dozens of APIs to help you achieve your goals.

Getting Started with ast-query

To start using ast-query, you need to install it via npm:

npm install ast-query

Useful API Methods in ast-query

Here, we will showcase some of the most useful API methods provided by ast-query along with code snippets to demonstrate their usage.

1. Loading AST

To manipulate an AST, you first need to load it:

const fs = require('fs');
const astQuery = require('ast-query');

const code = fs.readFileSync('example.js', 'utf8');
const ast = astQuery.parse(code);

2. Finding Nodes

You can query nodes within the AST:

const functionNodes = ast.find('FunctionDeclaration');

3. Modifying Nodes

ast-query allows you to easily modify nodes:

functionNodes.attr('type', 'FunctionExpression');

4. Generating Code from AST

After modifying the AST, you can generate code from it:

const modifiedCode = ast.toSource();
fs.writeFileSync('modifiedExample.js', modifiedCode);

Advanced Examples

Let’s take a look at an advanced example where we combine multiple API calls to refactor a function declaration:

const ast = astQuery.parse(fs.readFileSync('example.js', 'utf8'));

// Find function declarations
const functions = ast.find('FunctionDeclaration');

functions.each(function() {
  const node = this;

  // Rename the function
  node.attr('id.name', 'newFunctionName');

  // Convert to arrow function
  const arrowFunction = astQuery.arrowFunction(node.attr('params'), node.attr('body'));
  node.replace(arrowFunction);
});

// Write the modified code back to a file
fs.writeFileSync('refactoredExample.js', ast.toSource());

Building an Application with ast-query

Here is a simple example of an application that uses ast-query to perform automatic code refactoring in a codebase:

const fs = require('fs');
const path = require('path');
const astQuery = require('ast-query');

const directoryPath = path.join(__dirname, 'src');

fs.readdir(directoryPath, function(err, files) {
  if (err) {
    return console.log('Unable to scan directory: ' + err);
  }

  files.forEach(function(file) {
    const filePath = path.join(directoryPath, file);
    const code = fs.readFileSync(filePath, 'utf8');
    const ast = astQuery.parse(code);

    // Perform refactoring: rename functions, etc.
    const functions = ast.find('FunctionDeclaration');
    functions.each(function() {
      const node = this;
      node.attr('id.name', 'newFunctionName');
    });

    const modifiedCode = ast.toSource();
    fs.writeFileSync(filePath, modifiedCode);
    console.log(`Refactored ${file}`);
  });
});

With this simple application, you can automatically refactor function names in your JavaScript project.

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