We're not just setting the age to 0, we're setting this.age to 0. The same is true for this.name , this.title , etc. You can pass parameters into constructors. In order for your methods and constructor to work, which employee you're dealing with must be clear. Showing you methods might help make things more clear.
Methods are just functions attached to objects.
It's definitely a little weird and takes some time to get used to it. But once you start thinking in terms of objects, it soon becomes habit. The last step to building the object is to tie the method to the object. As I mentioned earlier, you do this inside the constructor. Now on to more complicated and important object-oriented JavaScript, like creating the virtual pet.