How to Specify Return Type in TypeScript Arrow Functions ?

To Specify the return type in the TypeScript arrow function, we have multiple approaches. In this article, we are going to learn how to Specify the type in the TypeScript arrow function.

Below are the approaches used to Specify the return type in the TypeScript arrow function:

Explicit Return Type Annotation

You explicitly annotate the return type after the parameter list using ':'

Example: Here, the add the function takes two parameters of type number and returns a value of type number.

Javascript

const add = (a: number, b: number): number => {
    return a + b;
};

console.log(add(3, 4)); // Output: 7

Output:

7

Type Inference

TypeScript infers the return type based on the return statement when the function body is a single expression.

Example: In this case, TypeScript infers that the return type is number based on the multiplication operation.

Javascript

const multiply = (a: number, b: number) => a * b;

console.log(multiply(5, 6)); // Output: 30

Output:

30

Function Declaration

You can specify the return type when declaring the function separately.

Example: You explicitly declare the function type before the assignment, specifying the return type.

Javascript

const divide: (a: number, b: number) =>
    number = (a, b) => {
        return a / b;
    };

console.log(divide(10, 2)); // Output: 5

Output:

5

Interface with Function Signature

An interface defines a function signature, and you use it to declare the arrow function.

Example: An interface defines a function signature, and you use it to declare the arrow function.

Javascript

interface MathOperation {
  (a: number, b: number): number;
}

const subtract: MathOperation = (a, b) => a - b;

console.log(subtract(8, 3)); // Output: 5

Output:

5

Using Type Alias for Function Signature

Type aliases in TypeScript provide a way to give a name to a type. You can use type aliases to define the signature of the function and then use it to specify the return type of the arrow function.

Example: In this example we defines a type alias MathOperation for a function taking two numbers and returning a number. It's assigned to a function power, calculating the exponentiation of 2 to 3.

JavaScript

type MathOperation = (a: number, b: number) => number;

const power: MathOperation = (base, exponent) => Math.pow(base, exponent);

console.log(power(2, 3)); 

Output:

8