Reason
TypeScript provides ReturnType to infer what type a function returns, but it isn't as as useful when functions return a promise and you want to know what type the promise resolves to.
Proposal
ResolvedType which returns the type of a function that returns a promise, but also works if the function is not a promise.
Example
// Silly simple example
const asyncFunction = async (n: number) => n * 2;
// We have this today
type AsyncFunctionReturns = ReturnType<typeof asyncFunction>;
// => Promise<number>
// This is the proposal
type AsyncFunctionReturns = ResolvedType<typeof asyncFunction>;
// => number
Proof of concept
Not tested extensively, may have bugs!
// For any function, return the returnType or promiseResolved type.
export type ResolvedValue<FuncPromOrValue> = FuncPromOrValue extends (...args: any) => any
? ReturnType<FuncPromOrValue> extends Promise<infer Result> // Is a function.
? Result // Is a promise, return result from Promise.
: ReturnType<FuncPromOrValue> // Non-promise function, return result.
: FuncPromOrValue; // Not a promise or function, return type.Existing work
I haven't seen anything like this before, but that doesn't mean it doesn't exist. I'm not fluent with other typed languages to know if there's a reason this doesn't exist or better way to implement it.
Reason
TypeScript provides ReturnType to infer what type a function returns, but it isn't as as useful when functions return a promise and you want to know what type the promise resolves to.
Proposal
ResolvedTypewhich returns the type of a function that returns a promise, but also works if the function is not a promise.Example
Proof of concept
Not tested extensively, may have bugs!
Existing work
I haven't seen anything like this before, but that doesn't mean it doesn't exist. I'm not fluent with other typed languages to know if there's a reason this doesn't exist or better way to implement it.