Get the return Type of a Function in TypeScript

# Get the return Type of a Function in TypeScript

Use the ReturnType utility type to get the return type of a function in TypeScript.

The ReturnType utility type constructs a type that consists of the return type of the provided function type.

index.ts

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function sum(a: number, b: number): number {
  return a + b;
}

// 👇️ type SumReturnType = number
type SumReturnType = ReturnType<typeof sum>;

// ✅ From a function's type
type AFunctionType = (a: number, b: string) => string;

// 👇️ type AFunctionReturnType = string
type AFunctionReturnType = ReturnType<AFunctionType>;

get return type of function in typescript

The code for this article is available on GitHub

We used the ReturnType utility type to get the return type of a function.

When trying to get the return type of a function from a function declaration, you have to use the typeof operator.

index.ts

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function sum(a: number, b: number): number {
  return a + b;
}

// 👇️ type SumReturnType = number
type SumReturnType = ReturnType<typeof sum>;

This is because ReturnType takes a type and not a function.

On the other hand, if you need to get the return type from a function's type, pass it directly to ReturnType.

index.ts

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// ✅ From a function's type
type AFunctionType = (a: number, b: string) => string;

// 👇️ type AFunctionReturnType = string
type AFunctionReturnType = ReturnType<AFunctionType>;

Notice that we didn't use the typeof operator because we are passing a type to the ReturnType utility type.

# Functions that don't return a value return `void`

If the function you're working with doesn't return a value, its return type will be void.

index.ts

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function myFunction(a: number, b: number): void {
  console.log(a);
  console.log(b);
}

// 👇️ type T = void
type T = ReturnType<typeof myFunction>;

functions that dont return value return void

The code for this article is available on GitHub

# Functions that might return multiple types return a union type

If the function might return values of multiple types, its return type will be a union type.

index.ts

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function myFunction(a: number, b: number): string | number {
  return Math.random() > 0.5 ? a + b : 'bobbyhadz.com';
}

// 👇️ type T = string | number
type T = ReturnType<typeof myFunction>;

returning union type from function

# Functions that throw an error return the type `never`

If the function throws an error, its return type will be never.

index.ts

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function myFunction(): never {
  throw new Error('Something went wrong');
}

// 👇️ type T = never
type T = ReturnType<typeof myFunction>;

The code for this article is available on GitHub

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

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Get the return Type of a Function in TypeScript