You definitely want to know and understand a lot of the basics and core principles of the language. You should be able to declare variables, understand data types, understand arrays, objects, declare functions, manipulate conditions and create loops. A very good understanding of these is something you definitely want to have before you learn a framework.
But, there's more.
Yes! There's more and below are my best picks for concepts I feel you should understand better before delving into a framework.
Most modern frameworks would make you work a lot with Objects and it's perhaps something you do not want to ignore and something you really want to wrap your head around.
You will find that most modern frameworks make the most of these. And if you lack an understanding of how it works, you may just on the surface know how to use it, but would lack a very fundamental "know-how" of how this works under the hood. And I feel it's more beneficial to know how it works than just how to use it.
I'm writing this with a smile on my face cause as of now I am still trying to understand this subject myself. But that said, it's something that is very common in modern frameworks and something that should be learnt and properly understood before even writing a single line of code that is dependent on a framework.
I could say a whole lot more about this, but I feel the above are the most important things you should learn before choosing a framework.