Generic Type for Promise in Typescript

Question

Generic Type for Promise in Typescript

I have a sample Promise function that is shown below. When successful, it returns a number, and when unsuccessful, it returns a string. The compiler is asking me to specify some kind of generic type for the promise. In this scenario, what type should I specify? Should I use Promise<number> or Promise<number | string>?

function test(arg: string): Promise {
    return new Promise((resolve, reject) => {
        if (arg === "a") {
            resolve(1);
        } else {
            reject("1");
        }
    });
}

generics typescript

Answer 1

Answer №1

The type specified in the Promise should align with the expected return data type of the function, while the error type is assumed to be any and not explicitly declared within the Promise definition.

For instance:

function example(input: string): Promise<number> {
    return new Promise<number>((resolve, reject) => {
        if (input === "x") {
            resolve(10);
        } else {
            reject("Error occurred");
        }
    });
}

Answer 2

The type specified in the Promise should align with the expected return data type of the function, while the error type is assumed to be any and not explicitly declared within the Promise definition.

For instance:

function example(input: string): Promise<number> {
    return new Promise<number>((resolve, reject) => {
        if (input === "x") {
            resolve(10);
        } else {
            reject("Error occurred");
        }
    });
}

Answer 3

Answer №2

If you want to create a specialized Promise type that pays attention to the type of rejection as well, you can define a custom Promise type. Alternatively, you can simply assign it one type and have the reject type default to any, similar to a standard 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 has type `number`
myPromise.catch((reason) => {}); //reason has type `string`

Answer 4

If you want to create a specialized Promise type that pays attention to the type of rejection as well, you can define a custom Promise type. Alternatively, you can simply assign it one type and have the reject type default to any, similar to a standard 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 has type `number`
myPromise.catch((reason) => {}); //reason has type `string`

Answer 5

Answer №3

const sampleFunction = (argument: string) => {
   return new Promise((resolve: (result: number) => void, reject) => {
    if (argument === "a") {
        resolve(1);
    } else {
        reject("1");
    }
  });
}

Answer 6

const sampleFunction = (argument: string) => {
   return new Promise((resolve: (result: number) => void, reject) => {
    if (argument === "a") {
        resolve(1);
    } else {
        reject("1");
    }
  });
}

Generic Type for Promise in Typescript

Answer №1

Answer №2

Answer №3

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