Suggestion: disallow implicit return of undefined

[neilgalarneau]

It would be useful to be able to disallow an Explicitly Typed function/method from implicitly returning the value undefined.

One example function that would be detected and disallowed was given by @rolyp in #2176. In that example, running off the end of the function causes undefined to be returned implicitly.

Another example that should be caught is:

function u(): number {
  return;
}

A flag for enabling this option could be named --noImplictReturnUndefined.

This suggestion fits in with the meta-bug in #274 and maybe #2176.

[danquirk]

Definitely fits well within either of those two larger options.

[RyanCavanaugh]

We should use this as an opportunity to look at code patterns in the wild -- how many JavaScript|TypeScript functions "intentionally" use an expressioned return and unexpressioned return in the same function body, especially in those with return type annotations.

[neilgalarneau]

I understand that TypeScript's goal is to be a super set of JavaScript, so legal JavaScript is legal TypeScript.

However, if I add types to my code, it is no longer JavaScript, and I would like TypeScript to help me fulfill the contract that the type usage represents.

If I declare a return type on my function and that function ends with "return;", I would like TypeScript to see that as an inconsistency & point that out to me.

[mhegazy]

--noImplicitReturns will flag missing a return statement. the main issue today is that undefined is a valid value for any type, and return; is equivalent to return undefined; i would say this falls under the bucket of #185