When you make the decision to start learning JavaScript it may be tempting to pick a JavaScript library that already contains a huge amount of extra functionality and to learn how to use that library to achieve all of what you are trying to do with JavaScript.
The problem with that approach is that you will never really learn JavaScript itself if you jump into using a library too soon. Sure the library will do a lot of things for you making the code that you need to write much simpler and quicker to write but you will be building a dependence on using that library even where the library isn't needed.
I have seen situations where someone asks for help with a single line of JavaScript code that isn't working quite correctly where they get replies that not only involve replacing the one line with two or three (which may actually be needed to fix the problem) but their solution also invokes a JavaScript library in running those two or three lines of code. That's a bit like someone saying "my toy boat doesn't float properly" and someone replying "here try this Ocean Liner instead".
There is nothing wrong with using a JavaScript library when you are writing a significant script that actually needs a decent fraction of what the library provides. It is somewhat ridiculous to use a JavaScript library (which all tend to be rather large due to all the processing they contain) when a few lines of JavaScript can do it without needing the library at all.
When you always use a JavaScript library when writing your code you don't get to learn how to do any of the processing that the library provides for you without using the library and so you end up needing to include the entire library when a small change to your code would do away with the need for it.The most obvious example of this is the $() function that most of the libraries provide which in most cases is just a shortcut way of saying document.getElementById() and if you don't know that you end up including the entire library even if that one call is all you really need. With this particular example just the code you need to add to include the library is longer than the code you are saving by using it and that's before you start to load the library at all.
The thing is that everything that the library provides is written in JavaScript and is only one of the ways that the particular task can be achieved in JavaScript (since other libraries probably use different code to achieve the same effects).
With a sufficient understanding of JavaScript itself you can choose when to use a library and when not to. Where the functionality that you require for a site is only going to use a small fraction of the library then there is no point in using the library and you should just incorporate the code that you need to do what the site requires. That may involve just pulling out a few functions from your favourite library to use them but since the libraries often have dependencies between the various pieces of code it may not be all that obvious which pieces you need and it may in fact be much easier to just write the processing you require for yourself. In order to do that though you need to have a sufficient knowledge of JavaScript to know what you need to write.
JavaScript libraries do provide a whole range of really useful functions to greatly enhance what you can do with JavaScript while dramatically reducing the amount of code you need to write yourself. Used properly they can make your script far more flexible as well as much easier to maintain. The libraries also take care of a lot of the differences between browsers for you so that the code you are writing will work regardless of which browser your visitors are using. You do need to have a reasonable understanding of what all of the functionality that the library contains is for and how it is intended to be used in order to be able to use the libraery most effectively and to be able to do that you really need to have a reasonable knowledge of how to do a lot of the things yourself without using the library.
Jumping into using a JavaScript library too soon after you first start to learn JavaScript means that you will not learn how to use JavaScript properly without the library and that will in turn limit your ability to use the library in the most effective way. It will also make it easier for you to figure out what the problem is when your script isn't working the way it is supposed to.