@codemod-utils/ast-javascript

@codemod-utils/ast-javascript

Utilities for handling *.{js,ts} files as abstract syntax tree

What is it?

@codemod-utils/ast-javascript wraps the methods from recast, a library that helps you parse and transform *.{js,ts} files.

The wrappers help you read and write files of different types in the same way. This way, you can focus on learning the builders and visit methods, the building blocks for transforming code (library-dependent).

import { AST } from '@codemod-utils/ast-javascript';

function transformCode(file, isTypeScript) {
  const traverse = AST.traverse(isTypeScript);

  const ast = traverse(file, {
    /* Use AST.builders to transform the tree */
  });

  return AST.print(ast);
}

API

In the traverse call, you can specify how to visit the nodes of interest ("visit methods") and how to modify them ("builders").

• Builders
• Visit methods

How to test your code

Currently, recast lacks documentation and tutorials. This is unfortunate, given the large amount of builders and visit methods that it provides to help you transform code.

I recommend using AST Explorer to test a small piece of code and familiarize with the API. The error messages from TypeScript, which you can find in your browser's console, can sometimes help. AST Workshop provides a good starting point for Handlebars.

If you intend to publish your codemod, I recommend using @codemod-utils/tests (create and test file fixtures) to check the output and prevent regressions.

AST Explorer for TypeScript

Select the following options to create a 4-tab window:

• Language: JavaScript
• Parser: recast
• Transform: recast

Copy-paste the visit methods from your file to AST explorer, then rename AST.builders to b.

Example

/* Your file */
import { AST } from '@codemod-utils/ast-javascript';

export function transformCode(file) {
  const traverse = AST.traverse(true);

  const ast = traverse(file, {
    visitClassDeclaration(path) {
      const { body } = path.node.body;

      const nodesToAdd = [
        AST.builders.classProperty(
          AST.builders.identifier('styles'),
          AST.builders.identifier('styles'),
        ),
      ];

      body.unshift(...nodesToAdd);

      return false;
    },
  });

  return AST.print(ast);
}

/* AST Explorer */
export default function transformer(code, { recast, parsers }) {
  const ast = recast.parse(code, { parser: parsers.typescript });
  const b = recast.types.builders;

  recast.visit(ast, {
    visitClassDeclaration(path) {
      const { body } = path.node.body;

      const nodesToAdd = [
        b.classProperty(
          b.identifier('styles'),
          b.identifier('styles')
        )
      ];

      body.unshift(...nodesToAdd);

      return false;
    }
  });

  return recast.print(ast).code;
}

How to type your code

@codemod-utils/ast-javascript avoids re-exporting the types from recast. This is to prevent a change in their API from catastrophically affecting your code.

When you write a function that depends on their implementation, type what you don't own as unknown, then use @ts-ignore or @ts-expect-error as needed.

Most importantly, write tests to document the inputs and outputs of your codemod. When there is an API change, you can refactor code with ease and confidence.

Compatibility

• Node.js v16 or above

Contributing

See the Contributing guide for details.

License

This project is licensed under the MIT License.