Many front-end developers rush to call themselves “Senior” because they can build React apps, but how many truly understand the fundamentals of JavaScript?

After interviewing over 100 so-called “Senior” devs, I’ve found that too many struggle with basic concepts like closures, array methods, and loops. If you can’t explain why your JavaScript code works (or doesn’t), are you really a Senior Developer? What's your take and easy way to filter them out before reaching the code-pairing?