function className() {} and var className = function(){} represent the same thing. See the examples below:
function Person(name, age){
        this.name = name;
        this.age = age;
     }
    Person.prototype.howOld =function(){
        console.log(this.name + " is " + this.age + " years old")
    }
    var person1 = new Person("John", 29);
    person1.howOld(); //John is 29 years old
The above would give you the same output as the below:
var Person = function(name, age){
    this.name = name;
    this.age = age;
 }
Person.prototype.howOld =function(){
    console.log(this.name + " is " + this.age + " years old")
}
var person1 = new Person("John", 29); 
person1.howOld(); //John is 29 years old
UPDATE: If you look at the two from a purely functional point of view, you might want to  consider something called hoisting. The above is just to show their similarity from a class/object point of view.