Enable `allowJs` with `isolatedDeclarations`

[weswigham]

Acknowledgement

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Comment

Today, an error is issued when allowJs is set alongside isolatedDeclarations - js and ts .d.ts emit use the same pipeline, so it really should work, provided the error generation logic isn't making too many syntax assumptions.

Mostly, unlocking this is going to involve looking through the isolatedDeclarations tests and copying them with their cast/annotation syntaxes swapped to jsdoc and filenames swapped to .js just to give confidence things are going to work OK (and that getEffectiveTypeNode is being used over a direct .type get in enough places in the error generator). There's a few other JS-specific error cases to look at (js's unique export/expression merges, for one) that may need some new errors, but the bulk of it is just ensuring the existing type node presence checking logic generalizes to jsdoc-sourced type nodes.

[saram-aman]

@weswigham can you please assign the task to me, would share a PR!

[pokey]

Probably not the same issue, but I get an error even if I inherit allowJs from a base config and then explicitly set it to false. I would assume the error should only fire if allowJs is set to true?

[rauschma]

Is there a reason not to always allow isolatedDeclarations? I find the additional constraints useful regardless of whether or not I emit declaration files.

[jakebailey]

At the implementation level, isolatedDeclarations diagnostics are extra declaration diagnostics produced by the declaration transformer, which we only run when declaration is enabled.

Theoretically it could be implemented such that isolatedDeclarations enables those checks (the diagnostics actually come from us running the transformer and then throwing away the resulting AST), but it is a change from the original design.

[samuelstroschein]

Here is a use case for allowJs in parallel to isolatedDeclarations opral/inlang-paraglide-js#160