Implicit `any`s with `Map` even if `noImplicitAny` is enabled

[Daniel15]

TypeScript Version: vNightly in Playground

Search Terms:
inferred any, implicit any, unsound

Code

function getStrings(): Map<string, string> {
  const val = new Map();
  val.set(42, true);
  return val;
}

Expected behavior:
I expected this to throw an error if noImplicitAny is enabled.

For what it's worth, Flow handles this correctly.

Actual behavior:
No errors

Playground link

Related Issues:
No

[RyanCavanaugh]

Duplicate #33279

Search term: noImplicitAny Map

[Daniel15]

@RyanCavanaugh So is this actually expected behaviour? This seems like a big hole in TypeScript's typing.

Flow gets this right.

[krisdages]

I've also been burned by this kind of thing before; usually when RXJS pipe inference gets widened to any without my realizing it.

Even if noImplicitAny is working as expected,

A stricter compile option that would force assertions before passing or assigning something with an inferred any Type Parameter to an explicitly narrower type would be immensely helpful.

This example with new Map() is relatively simple, but in more complex inference scenarios (e.g rxjs pipe chaining or third party definitions whose generics and defaults you don't necessarily know), it's difficult to tell when you have lost type safety, but the error potential is huge.

---

With an option like I am imagining, in the OP's example there would be no error in creating the Map or setting the values, but it would be an error to return it as Map<string, string> unless the constructor had explicitly been called with <any, any>

[krisdages]

Another source of these unexpected "any" bugs is when a conditional type resolves to "any", often unexpectedly.

I have looked into things like dtslint to try and catch these, but they have been sort of a square peg / round hole situation.

If there are any alternatives to a compiler option that could alert me when any is being used when the declaration says something else, I would love to find out about them. (Sincerity quotes, no sarcasm)

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.