I don't understand why chaining focus() and keyup() is actually working

jQuery allows you to chain some methods together. focus() takes a function as an input and will run that function when the user brings focus to the selected element. Like clicking in a text box for example. However, this method will only run once per focus. So if you need to execute the function again, you need to bring focus to the selected text box.

That's where the keyup() method comes in handy. It also takes a function as an input, but it only runs that function when the user stops typing. So in the chaining example the user first brings focus to the password box. The function specified will then run. When the user stops typing the keyup() method is triggered and the specified function is run again. It's important to note that you do not have to chain these methods together. You could also do the following and still have the code work the same. It's just more code.

$("#password").focus(passwordEvent);
$("#password").keyup(passwordEvent);

So, if I understand correctly, the focus() method only runs once but it returns the jQuery object $("#password") which means effectively after the focus event is fired and the method is run, we end up with $("#password").keyup(passwordEvent);