@codemod-utils/ast-javascript

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

What is it?

@codemod-utils/ast-javascript provides methods from recast to help you parse and transform *.{js,ts} files.

How to update JavaScript

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

type Data = {
  isTypeScript: boolean;
};

function transform(file: string, data: Data): string {
  const traverse = AST.traverse(data.isTypeScript);

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

  return AST.print(ast);
}

API

AST

An object that provides builders, print, and traverse.

In a 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.

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

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

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

Once you are satisfied with the code, you can copy-paste the visit method(s) to your file, then rename b. to AST.builders..

Example (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;
}

Example (Your file)

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

export function transform(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);
}

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.