TypeScript Return Type Annotations

In this article, we are going to learn about the Return Type Annotations in Typescript. TypeScript is a popular programming language used for building scalable and robust applications. One of the features of Typescript is Return Type Annotations which helps to explicitly declare the type of value that a function is expected to return. Return type annotations are added to the function’s signature and come after the parameter list using a colon (:) followed by the desired return type.

Syntax:

function functionName(parameters: parameterType): returnType {
    // Body of function
}

Where-

• functionName is the name of the function.
• parameters is a list of function parameters with their types.
• returnType is the type that the function is expected to return.

Example 1: In this example, ‘multiply’ is the function name. ‘a’ and ‘b’ are parameters with type numbers. number specifies that the function is expected a number as a return value. We can change that return type according to our need e.g.: string, boolean, function, object, array, null, and void.

Javascript

function multiply(a: number, b: number): number {
    return a * b;
}
console.log(multiply(7, 2))

Output:

Example 2: In this example, we will see Promise as the return type. It indicates that the function returns a promise that resolves to a specific type.We define a function fetchData that returns a Promise<string>. This means that fetchData is expected to return a promise that will resolve to a string. We call fetchData() and then use the .then() method to handle the resolved value.

Javascript

async function fetchData(): Promise<string> {
    return "Hi Geek!! Data fetched successfully";
}
fetchData().then((result) => {
    // Output:Hi Geek!! Data fetched successfully
    console.log(result);
});

Output: