let's see... Lack of a real integer type ?
The fact that {} + [], [] + [], [] + {} are all valid and all give different results ? [1]
The fact that it has some silent error conditions like NaN ?
The fact that, in v8 (which is the most common js engine out there), if you add a comment to a function, it can suddenly start performing poorly ?[0]
I love JS, I really do. Hell, I write nodejs for a living ! I find prototypical inheritance to be an awesome idea. The simplicity, and extensibility of the language is something that I really like from it. But I can see where people hating it are coming from. I wish lua had been used instead of JS.
[0]: https://top.fse.guru/nodejs-a-quick-optimization-advice-7353...