"duplicate identifier" error in js files produced by tsc

[d180cf]

Let's say we have the following ts file:

function abc() {
  // ...
}

module abc {
  export const def = 123;
}

tsc converts this into the following js file:

function abc() {
  // ...
}

var abc;
abc.def = 123; // well, there is actually a function (abc) { ... } wrapper here

Now if I try to include this js file into another ts file, tsc will scan the js file it's just generated and will report an error: "duplicate identifier abc". Once the "var abc" part is removed, the error goes away. Tried with the latest typescript@next (which was 1.9.0-dev.20160214).

The good news is that this is the only error I'm getting from a exceptionally sophisticated js file.

[RyanCavanaugh]

We could do the work to not emit the var when we know we've already emitted the function. Note that this isn't going to solve every case since you might have these declarations in separate files.

[d180cf]

I'm actually thinking that emitting the error isn't correct in this case: an empty "var" should be a noop if the var was already declared.

[mhegazy]

@d180cf as per the JS semantics, the additional var is a no-op. the typescript compiler imposes a bit stricter semantics here; the assumption here is that you did that be mistake and you did not mean to re-declare the variable, and that might point you to an actual bug, either related to scoping or incorrect use.
I agree with @RyanCavanaugh that the compiler output should hold up to its own check.

[basarat]

If one uses outDir and exclude options wouldn't this be avoided altogether 🌹

[mhegazy]

PRs welcomed!

[RyanCavanaugh]

@mhegazy we should probably hold off on PRs for this until after the tree transforms work comes in? Don't want to solicit throwaway work

[mhegazy]

that is correct. here is the issue to look out for: #5595

[RyanCavanaugh]

This is fixed