error when a dataloader has a single extension

[Fil]

this seems like good practice:

• matches the documentation
• warns the user, and prevents outputting payloads with no extension
• slightly increases security (if your dev server is public, it won't be used to run ANY .sh file in your docs directory, these files now need a double extension)

[mbostock]

Good catch!

To confirm my understanding, the bug is: the user requested the extension-less file foo; this file did not exist, so we then (inadventantly) checked for a single-extension data loader such as foo.js (etc.), and found it. But we intended that data loaders should have a double extension, so we should not have considered foo.js a data loader.

I think the fix here is not to error but to return undefined above the for loop. The proposed error implies that the user intended for foo.ts to be a data loader, but that’s not necessarily true and I think a 404 is more expected and neutral: the user asked for foo and it didn’t exist, and it was just a bug in our implementation that we considered foo.ts a data loader.

[Fil]

I came to it the other way (by writing a data loader without a double extension), but I agree that the scenario you're mentioning is more common. We can add a log, though, to help in both cases.