I'm struggling to fully grasp the definition of Promise in TypeScript, as shown below:
/**
* Represents the completion of an asynchronous operation
*/
interface Promise<T> {
/**
* Attaches callbacks for the resolution and/or rejection of the Promise.
* @param onfulfilled The callback to execute when the Promise is resolved.
* @param onrejected The callback to execute when the Promise is rejected.
* @returns A Promise for the completion of which ever callback is executed.
*/
then<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null): Promise<TResult1 | TResult2>;
/**
* Attaches a callback for only the rejection of the Promise.
* @param onrejected The callback to execute when the Promise is rejected.
* @returns A Promise for the completion of the callback.
*/
catch<TResult = never>(onrejected?: ((reason: any) => TResult | PromiseLike<TResult>) | undefined | null): Promise<T | TResult>;
}
My understanding is that
then<TResult1 = T, TResult2 = never>then has two generic types, namely TResult1 and TResult2. If not specified, TResult1 defaults to T.
However, it seems that TResult1 is determined by the return type of onfulfilled. Consider this demo:
interface Result {
status: number;
message: string;
}
function foo() {
return new Promise<Result>(function (resolve, reject) {
resolve({
status: 0,
message: 'ok',
});
});
}
// here fulfilled1's type is: (local function) fulfilled(out: Result): number
foo().then(function fulfilled1(out) {
if (Math.random() > 0.5) {
return 1;
}
});
// here fullfilled2's type is: (local function) fulfilled2(out: Result): string
foo().then(function fulfilled2(out) {
if (Math.random() > 0.5) {
return 'hello';
}
});
Both fulfilled1 and fulfilled2 fit into
then<TResult1 = T, TResult2 = never>then, I expected TResult1 to be Result, but it actually becomes number and string in these cases.
It seems like I may have misunderstood something about TypeScript generics. Any insights would be greatly appreciated.