How to Strongly Type the Return Value of any Function in TypeScript ?

In TypeScript, specifying the return type after a function’s parameters with a colon ensures a consistent return value type. This enhances code reliability by enforcing expected return types. It is a fundamental feature of TypeScript that ensures functions consistently produce values of the specified type. The below methods can be used to achieve this task:

Directly specifying the return type

You can specify the type of the return value of a function directly by using the colon(:) syntax followed by the return type.

Syntax:

function functionName(): returnType{}

Example: The below code will show how you can directly type the return value of a function.

Javascript

function sum(a: number, b: number): number {
  return a + b;
}
const result: number = sum(15, 30);
console.log(result);

Output:

45

Using Generic type

The generic type can be used to declare the functions with a specified return as well as accepting values. It will return the value of the same type which it takes.

Syntax:

function functionName<T>(): T{}

Example: The below code will explain the use of generic type to type return value of a function.

Javascript

function genericInput<T>(input: T): T {
    return input;
}
let noResult: number =
    genericInput<number>(10);
let strResult: string =
    genericInput<string>("Geek");
console.log("Number result:", noResult);
console.log("String result:", strResult);

Output:

Number result: 10
String result: Geek

Using TypeScript Interfaces

You can also use TypeScript interface to create a blue print of the return type value of a function which can be later used to assign the return type to the function.

Syntax:

interface interfaceName{}

Example: The below code practically implements the interface to type the return value of the function.

Javascript

interface ReturnValue {
    result: number;
}

function myFunc(): ReturnValue {
    return { result: 10 };
}

const result: ReturnValue = myFunc();
console.log("Result:", result.result);

Output:

Result: 10