as far as i know
learn the data structure and their respective helping provided methods to help you for different things like of the String , Array in javascript.
then use of the class is just to do the object oriented programming like making the objects (our own defined) with our own defined methods either using the preprovided for the particular data structure(String, Array) or the newly created data structure and their methods like (stack, queue)
built in data structure (primitive and nonprimitive )

note this thing usually follows in the web development
web development is all about
educating yourself with the concepts like
frontend, backend, their communication way (http, graphql), cors, security, passing the different type of data like text and file through the form and taking it from the backend and responsing as per the query or the input data.
Here in the backend development developers use to define the classes for differenct scenerios like creating, updating, deleting, uploading(file), etc. and using the high order functions for the utilities purpose.
and using the pure javascript for defining our own required functionality for the particular problem.
also dont memorize the stuffs instead learn the concepts.

[THEIR COULD BE SYNTAX ERROR BUT SEND YOUR FEEDBACK ALSO IT IS MY FIRST POST 😀]