Declare a function with a Promise return type in TypeScript

Declare a function with a Promise return type in TypeScript #

To declare a function with a promise return type, set the return type of the function to a promise right after the function's parameter list, e.g. function getPromise(): Promise<number> {}. If the return type of the function is not set, TypeScript will infer it.

index.ts

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// ✅ Named function
function getPromise(): Promise<number> {
  return Promise.resolve(5);
}

// 👇️ Unwrap promise type if necessary
// 👇️ type T = number
type T = Awaited<ReturnType<typeof getPromise>>;

// ✅ Named async function
async function getPromise2(): Promise<number> {
  return 10;
}

// ✅ Arrow function
const getPromise3 = (): Promise<string> => {
  return Promise.resolve('hello');
};

// ✅ Class method
class A {
  async getPromise(): Promise<string> {
    return 'hello world';
  }
}

// ✅ Using a type as Promise<Type>
type Person = {
  name: string;
  age: number;
};

async function getPromise4(): Promise<Person[]> {
  return [
    {
      name: 'Tom',
      age: 30,
    },
  ];
}

You can set the return type of a function right after its parameter list.

index.ts

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function getPromise(a: number): Promise<number> {
  return Promise.resolve(a);
}

If you need to unwrap the type of the Promise from a function's return type, use the Awaited utility type.

index.ts

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function getPromise(a: number): Promise<number> {
  return Promise.resolve(a);
}

// 👇️ Unwrap promise type if necessary
// 👇️ type T = number
type T = Awaited<ReturnType<typeof getPromise>>;

The Awaited utility type recursively unwraps a promise and returns its type.

index.ts

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// 👇️ type A = string
type A = Awaited<Promise<string>>;

The examples above show how to set the return type of a function to a promise containing different values.

The advantage of setting the return type of the function explicitly is that the type checker would throw an error if you try to return a promise of a different type.

index.ts

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async function getPromise2(): Promise<number> {
  // ⛔️ Type 'string' is not assignable
  // to type 'number'.ts(2322)
  return 'hello';
}

If you don't explicitly set the return type of the function, TypeScript will infer it.

index.ts

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// 👇️ function getPromise2(): Promise<string>
async function getPromise2() {
  return 'hello';
}

If your function returns a Promise that may contain different values, use a union to specify the function's return type.

index.ts

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async function getPromise2(): Promise<string | null> {
  if (Math.random() > 0.5) {
    return null;
  }

  return 'success';
}

The function above returns a Promise that contains a string or a null value.

If your function returns a Promise that unwraps to an array of objects, it's more readable to use a type or an interface.

index.ts

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type Person = {
  name: string;
  age: number;
};

async function getPromise4(): Promise<Person[]> {
  return [
    {
      name: 'Tom',
      age: 30,
    },
  ];
}

The function above returns a Promise that contains an array of objects of type Person.

Declare a function with a Promise return type in TypeScript

Declare a function with a Promise return type in TypeScript #

Borislav Hadzhiev