Editing the title because it took me a moment to figure this out. 😄

I tried to fix this issue! See #9121.
Below is a screenshot of the result.

• If the file is ESM
  

• If the file is not ESM
  

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In /tyscript-estree/src/convert.ts/Converter/convertNode, modifying sourceType when it is case SyntaxKind.SourceFile does not make sense, below is the detail.

I tried modifying the sourceFile in convertNode with extension === ts.Extension.Mjs || extension === ts.Extension.Mts, but only the AST root (SourceFile) is changed, not the multiple child Nodes.

It appears that before convertNode is called, the ts AST is fully constructed via ts.createSourceFile and then changed to the estree AST via convertNode.

In /tyscript-estree/src/create-program/createSourceFile.ts/createSourceFile, adjusting impliedNodeFormat, parameter of the ts.createSourceFile, doesn't work, below is the detail.

The Interface annotation says "Controls the format the file is detected as", so I was expecting that, but modifying the impliedNodeFormat didn't achieve the desired result. It seems to be a parameter related to js Emit, not AST construction.

Hi @fisker!

I updated the PR to find a new way to do this because my original suggestion was unacceptable, and the new way I found was to modify setExternalModuleIndicator.

The reason I initially thought it was inappropriate to modify setExternalModuleIndicator was because I was concerned that it would be too difficult to design a new function based on the behavior of the default(but we didn't have to), and that it would be too dependent on external modules.

However, while analyzing the typesciprt code to find a new way, I found that it is very easy to create an externalModuleIndicator that simply returns true unconditionally based on the file extension, and I also found cases where the same thing was done in the typescript internal code.

See PR's comment for more details.