Let me be upfront about something, I made every single mistake on this list. Some of them multiple times. One of them I made in a codebase that was actually live and serving real users, which was a fun afternoon of emergency debugging. This isn't a "beginners are bad" post. It's more of a "here's the stuff that tripped me up and still trips people up, even when they feel like they should know better" post. JavaScript has some genuinely weird behaviors baked into it, and a lot of beginners get burned by the same things repeatedly. Let's go through them. Mistake 1: Still Using var in 2026 I still see var in fresh codebases. Not legacy code, brand new projects, written recently, by people who've learned from modern tutorials. Somewhere the message hasn't fully landed. The problem with var isn't just stylistic. It's hoisted to the top of its function scope, which creates bugs that are legitimately confusing to trace, especially inside loops or conditional blocks. Use const by default.…