Today a front end designer needs to read 50 documents to understand CSS, not least be able to make a small change to a web site without it all falling down like a pile of Jenga blocks. That makes coding a fast suffix tree building algorithm out of your head look easy.
In the case of React you could ask questions like "why does this simple web page call the render() method 714 times?" and then the interviewee says "that's wrong, it calls render() 715 times" and they will have to bring in one of the React core developers to adjudicate.