Promise being resolved with wrong type

[0xfede]

TypeScript Version: 1.8.10 and 2.0.0 beta

Code

function getPromise(): Promise<string> {
  return new Promise(resolve => {
    resolve(1234); // Should fail as 'abcd' is not a number
  });
}

getPromise().then(s => {
  console.log(s.length);
});

Expected behaviour: A compile error on line 3

Actual behaviour: The program prints undefined

If I copy the type definition of resolve from the interface PromiseConstructor (in lib.es6.d.ts), I get the expected behaviour:

function getPromise(): Promise<string> {
  return new Promise((resolve: (value?: string | PromiseLike<string>) => void) => {
    resolve(1234); // As expected I get an error
    // TS2345: Argument of type 'number' is not assignable to parameter of type 'string | PromiseLike<string>' Type 'number' is not assignable to type 'PromiseLike<string>'.
  });
}

getPromise().then(s => {
  console.log(s.length);
});

[jeffreymorlan]

It's easier to specify the type argument: new Promise<string>(resolve => {...})

Unfortunately, new Promise(resolve => {...}) without any type annotations always has a type of Promise<{}> due to #5254, which is assignable to any other Promise due to #6407.

[badescuga]

I'm also seeing this :) Not sure why the type is not being respected. TS 2.6.2

type Dictionary<T> = { [id: string]: T };

 public getGamesById(gameIds: string[]): Promise<Dictionary<Game>> {
const games: Game[];
return Promise.resolve(games);
}

[waratah]

@badescuga any is a catch all and will totally break type safety. Got me last week. The more definitions you introduce in your code the better. You can also use noImplicitAny to pick up on these errors.

[RyanCavanaugh]

This has been fixed

[acashjos]

TypeScript Version: Version 3.6.3

Just a reminder, while the fix works for most practical cases, it still doesn't detect type inconsistencies for Promise<Object > , Promise<{}> or any interface defined as {}

interface Message { }
function getPromise(): Promise<Message> {
   return new Promise(resolve => {
       resolve(1234); // Should fail as 'abcd' is not a Message
   });
}

getPromise().then(s => {
   console.log(s.length);  // fails as length is not a property of Message
});