Philosophy
What is FE? Who are you? Is this for you? Why did you come to FE?
In my definition, FE is what you can see and touch. So if you work with FE, you need to create things beauty and easy to use. Then if you were an engineer, you need to know how to drive the car together with how the engine work.
Easier to understand, let talk about jQuery. A FE Engineer (FEE) will know when to use $.extends() and Object.assign(). What is running behind the $.show() and how to execute promise if you can not use $q?
I once asked a developer that if he need to implement two way binding to an existing jQuery app, what would he do? If you have never though about it, you still have time ;)