I've a sample Promise function like below. On success I return a number and on false I return string. The compiler is complaining to specify some kind of generic type to the promise. In this case what type I've to specify? Do I've to specify like Promise<number> or Promise<number | string>?
function test(arg: string): Promise {
return new Promise((resolve, reject) => {
if (arg === "a") {
resolve(1);
} else {
reject("1");
}
});
}
The generic type of the Promise should correspond to the non-error return-type of the function. The error is implicitly of type any and is not specified in the Promise generic type.
So for example:
function test(arg: string): Promise<number> {
return new Promise<number>((resolve, reject) => {
if (arg === "a") {
resolve(1);
} else {
reject("1");
}
});
}
Promise<T> builtin generic type? Thank you!
Promise has any error type. The error type can't be constrained because any running code within the promise can throw anything, not just normal Error. (See developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… . It's throw expression). Since the promise is a generic object which can't control what runs inside it, and it would also be tedious/over-expectant to expect you (the dev) to control what runs inside it, the error type can't be constrained.
Commented
Oct 7, 2020 at 13:53
You could define a custom Promise type that actually also cares about the type of the rejection. You can also just give it a single type and the reject type will be any, like in a normal promise.
type CustomPromise<T, F = any> = {
catch<TResult = never>(
onrejected?: ((reason: F) => TResult | PromiseLike<TResult>) | undefined | null
): Promise<T | TResult>;
} & Promise<T>;
function test(arg: string): CustomPromise<number, string> {
return new Promise((resolve, reject) => {
if (arg === "a") {
resolve(1);
} else {
reject("1");
}
});
}
const myPromise = test("a");
myPromise.then((value) => {}); //value is of type `number`
myPromise.catch((reason) => {}); //reason is of type `string`
function test(arg: string) {
return new Promise((resolve: (value: number) => void, reject) => {
if (arg === "a") {
resolve(1);
} else {
reject("1");
}
});
}
Promise<number|string>?