TIL: Powerful refactoring with ast-grep

I recently learned about a tool called ast-grep. It’s a powerful CLI tool that can find and replace bits of code based on their ASTs. In unix terms it’s a superior grep and/or sed for code. It’s backed by tree sitter giving it support for a dizzying number of languages.

This week I’m migrating an old codebase from Node 14 to 22. Some of the packages were using an old package esm that added support for ECMAScript modules before Node had native support for them. I had to remove the package because it doesn’t work on Node 22 anymore. As part of moving to Node’s native es modules I had to rewrite a bunch of imports to include the full path and extension. For example if we previously had import utils from './utils' it now needed to be import utils from './utils.js' because the old way was ambiguous.

Here’s an example showing how to find import statements for local files that start with ./ or ../:

language: js
rule:
  regex: "^[\\.]{1,2}/.*$"
  kind: string_fragment
  any:
    - inside:
        stopBy: end
        kind: import_statement

You can include a top level fix: property to perform replacements but I left it out of my example. If you do include a fix you can run the rule like this:

sg scan --interactive --rule some_rule.yaml

It’ll helpfully prompt you for each file modification.