Mastering TypeScript 3 - Third Edition

Function return types

Using the very simple syntactic sugar that TypeScript uses, we can easily define the type of a variable that a function should return. In other words, when we call a function, and it returns a value, what type should the return value be treated as?

Consider the following TypeScript code:

function addNumbers(a: number, b: number) : string { 
    return a + b; 
} 
var addResult = addNumbers(2,3); 
console.log(`addNumbers returned : ${addResult}`); 

Here, we have added a :number type to both of the parameters of the addNumbers function (a and b), and we have also added a :string type just after the ( ) braces in the function definition. Placing a type annotation after the function definition means that we are defining the return ...