Edit variables in a matched pattern

[goodov]

Is there a way to match part of code with meta-variables and let ast-grep rewrite the variables while keeping other parts of the matched code untouched? The use case for this is syntax-aware patching where I want to match the exact AST surrounding the code I want to patch.

The issue I'm facing right now is that I should rewrite the template as is to add something new in the fix part, instead of just modifying the variable.

Consider this example (playground link):

if (true) {
  const notThis = 'console.log("not me")'
  const a = getA();
  if (a == 333) {
    return false;
  }
  if (a == 444) {
    return true;
  }
}

I want to add new code after const a = getA(); line, but before if (a == 333) {. Right now to do this I had to write this config:

rule:
  pattern:
    context: |-
      if (true) {
        $$$
        const a = getA();
        $$$INSERTION_POINT
        if (a == 333) { $$$ }
        $$$
      }
    strictness: cst
fix: |-
  if (true) {
    const notThis = 'console.log("not me")'
    const a = getA();
    if (a == 111) {  // This is the new condition added in the "fix" block.
      return false;
    }
    if (a == 333) {
      return false;
    }
    if (a == 444) {
      return true;
    }
  }

As you can see, I had to mostly duplicate the matched code in the fix part. What I'd like to have is something like edit functionality:

edit:
  - replace: $$$INSERTION_POINT
    with: |-
      if (a == 111) {
        return false;
      }
      $$$INSERTION_POINT

Does ast-grep support this in some way? What are my options here? Any advice would be greatly appreciated!

[HerringtonDarkholme]

You can use rewriter and replace
https://ast-grep.github.io/guide/rewrite/rewriter.html