Get the return Type of a Promise in TypeScript

Get the return Type of a Promise in TypeScript #

Use the Awaited utility type to get the return type of a promise in TypeScript, e.g. type A = Awaited<Promise<string>>. The Awaited utility type is used to recursively unwrap Promises and get their return type.

index.ts

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

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

// 👇️ C = boolean | number
type C = Awaited<boolean | Promise<number>>;

async function sum(a: number, b: number): Promise<number> {
  return a + b;
}

// 👇️ type D = number
type D = Awaited<ReturnType<typeof sum>>;

We used the Awaited utility type to get the return type of a couple of Promises in the examples.

The type is used to recursively unwrap a Promise and get its return type.

The second example shows that we can unwrap the type of the promise even if it's nested.

index.ts

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

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

index.ts

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function multiply(a: number, b: number): Promise<number> {
  return Promise.resolve(a * b);
}

// 👇️ type E = number
type E = Awaited<ReturnType<typeof multiply>>;

The ReturnType utility type constructs a type consisting of the function's return type.

You can use the same approach to unwrap the return type of an async function. Note that async functions always return a Promise.

index.ts

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async function sum(a: number, b: number): Promise<number> {
  return a + b;
}

// 👇️ type D = number
type D = Awaited<ReturnType<typeof sum>>;

This approach also works with built-in methods like Promise.all and Promise.race.

index.ts

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async function getArr(a: number, b: number): Promise<[number, string]> {
  const result = await Promise.all([
    Promise.resolve(5),
    Promise.resolve('hello'),
  ]);

  return result;
}

// 👇️ [number, string]
type E = Awaited<ReturnType<typeof getArr>>;

Get the return Type of a Promise in TypeScript

Get the return Type of a Promise in TypeScript #

Borislav Hadzhiev