First, you should ask for his/her Github username and check public repositories.
Note: Keep in mind that many of these questions are open-ended and could lead to interesting discussions that tell you more about the person's capabilities than a straight answer would.
- What Javascript Frameworks do you know? Which one do you like? Why?
- What is the atomic design?
Coined by Brad Frost, Atomic Design is a methodology that involves breaking a website layout down into its basic components, which are then reused throughout the site. You have Atoms => Molecules => Organisms => Templates => Pages.
- Can you name two programming paradigms important for JavaScript app developers?