[dcherman]

This post is entirely going to be based on questions that I would expect an experienced developer that I was interviewing to be able to have a conversation on. This is not a starting point for a beginner.

Understand the language and how it works on a deep level. If there's ever a question of "should this work?", read the spec for that feature - it's generally not that hard to understand.

Understand the runtime. What is an event loop? What's a macro task? A micro task? How does each work? How does all of this integrate with the DOM as far as what might cause a dropped frame, block rendering, etc..?

Understand the ideas behind the framework you're using. I'm not going to recommend an individual one to avoid framework wars, but most of them embrace virtual dom these days, so you should have a basic concept of what that is. Does it make applications faster? Does it make it developing easier?. Another common framework point is unidirectional data flow; why is unidirectional data flow considered a good thing these days? What were the downfalls of bidirectional data flow?

Understand API design and state boundaries. Using the example of a shopping cart, who owns what? Where is the state of the cart stored? An individual item price? Name? How do I add an item to the cart? You can design a cart that functions fantastically, but is completely un-testable if the API is designed wrong or there is not well defined boundaries on what components own what state.

That's a few things off the top of my head.