Michal Ostruszka Michal Ostruszka - 6 days ago 6
TypeScript Question

Are promise type parameters checked in TypeScript?

Why doesn't the following example generate a TypeScript compiler warning (instead failing at runtime) when I try to work on

foo
as
Promise<string>
?

let foo: Promise<string> = Promise.resolve(10); // should not compile
foo.then(v => v.toUpperCase());

// -> TypeError: v.toUpperCase is not a function


Only if I put
Promise.resolve<string>(10)
do I get an error, but that should not be the required as the TypeScript compiler is smart enough to infer type of the RHS as
Promise<number>
when inspected. So why it doesn't yell at me when I try to do this assignment? Also, is there any workaround for that other than specifying type explicitly? I'm using TypeScript 2.0.9.

Answer

This appears to be a known issue (also see #10524, which is the main tracking issue for this problem) and should be fixed in v2.1.2 by the linked commit. You should be able to upgrade and the types will then be enforced properly, as intended.

If the issue persists in v2.1.x, my suggestion would be to report a bug in their issue tracker.

Comments