Issue about changing elements of a date and time syntax in JS

In chapter 34 of Smarter Way to Learn JS book, in this exercise:

Create a Date object for the current date and time.
1-Extract the year.
2-Reset the Date object a century back.
3-Display the date Object in an alert.

The solution is

1- var now = new Date();
2- var nowYear = now.getFullYear(); 
3- now.setFullYear(nowYear - 100);
4- alert(now);

What I did not understand is line 3. Why don’t we write this code instead:

now = now.setFullYear(nowYear - 100);

These two types of codes yeilds different results.

If I understood well the line 3 of the first example should be inherently written that way, I have just have to learn that syntax no need to overthink.

If you have a better analysis or an interesting link about this issue feel free to reply.

The new Date() returns an object which has several properties and methods. setFullYear is one of those methods. The dot syntax is used because now is an object.