Full control of build filename in library mode

[hronro]

Clear and concise description of the problem

vite/packages/vite/src/node/build.ts

Line 417 in 4afb743

? `${libOptions.fileName || pkgName}.${output.format || `es`}.js`

Currently vite always append .es.js in the end of filename in library mode, but there should be a way to let users fully control the filenames.

Suggested solution

Add another options in LibraryOptions like fullyControlledFileName:

interface LibraryOptions {
  // ...
  fileName?: string;
  fullyControlledFileName?: string | (format: LibraryFormats) => string;
}

fullyControlledFileName should have higher priority than fileName, so if fullyControlledFileName is not undefined, vite should ignore the fileName option.

Alternative

Another way is changing the current behavior of the option fileName to make user fully control how the file name is generated, but this would be a breaking change to those who relay on the current file naming strategy.

[sodatea]

We've discussed this issue in a team meeting before. Instead of adding a new option, I think letting fileName accept a function would suffice.