Returning a promise in an async function in TypeScript

Question

It's my understanding that these two functions will have the same behavior in JavaScript:

const whatever1 = (): Promise<number> => {     return new Promise((resolve) => {         resolve(4);     }); };  const whatever2 = async (): Promise<number> => {     return new Promise((resolve) => {         resolve(4);     }); }; 

But TypeScript seems to not like the second one, it says:

Type '{}' is not assignable to type 'number'. 

Is this a bug in TypeScript, or am I misunderstanding something about async functions?

Answer 1

It's complicated.

First of all, in this code

const p = new Promise((resolve) => {     resolve(4); }); 

the type of p is inferred as Promise<{}>. There is open issue about this on typescript github, so arguably this is a bug, because obviously (for a human), p should be Promise<number>.

Then, Promise<{}> is compatible with Promise<number>, because basically the only property a promise has is then method, and then is compatible in these two promise types in accordance with typescript rules for function types compatibility. That's why there is no error in whatever1.

But the purpose of async is to pretend that you are dealing with actual values, not promises, and then you get the error in whatever2 because {} is obvioulsy not compatible with number.

So the async behavior is the same, but currently some workaround is necessary to make typescript compile it. You could simply provide explicit generic argument when creating a promise like this:

const whatever2 = async (): Promise<number> => {     return new Promise<number>((resolve) => {         resolve(4);     }); };