Some JavaScript developers know the theory, but not all of them. At least you should know how it practically works.

So when you ask "What is the JavaScript event loop?", most will stare at you not knowing what it is. So then I jump to the practical side. They at least should have worked with either setTimeout, async/await and/or events. So then you start asking for practical consequences of the event loop, such as what happens with an infinite loop after setTimeout or await, or during handling of an event. That is a real practical example that they should know. If you don't then you don't know neither theory nor practice.