why does this return undefined - javascript

Question

i'm pretty new to js. i'm sorry if this sounds dumb. but why does the following code return "undefined"

function NewPerson(name, age, sex){
    this.name = name;
    this.age = age;
    this.sex = sex;

    this.getName = function(){
        //alert(this.name);
        alert("The age is "+this.age);
    };

}

var obj1 = new NewPerson("Mark",25,"Male");
alert("The age is as follows "+obj1.getName());

// output:

The age is 25 The age is as follows undefined

Answer 1

Because you don't return anything.