Most Important javascript concept before starting Reactjs

1.Arrow Functions:
An arrow function expression is a compact alternative to a
traditional function expression.
https://javascript.info/arrow-functions-basics

2.Let/var/const variables:
Understand the difference between them, uses and their
respective scopes.
https://youtu.be/BNC6slYCj50

https://www.freecodecamp.org/news/var-let-and-const-whats-the-difference/

3.Destructuring assignment:
JavaScript expression that makes it possible to unpack values
from arrays, or properties from objects, into a bunch of
variables.
https://youtu.be/giNjEgYTd9E
https://developer.mozilla.org/enUS/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment

4.Template Literals:
With ES6, a newer form of string called template literal was
given, which consists of two backticks .
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals

https://youtu.be/K4Kh5gw4PRE

5.Using Fetch:
The Fetch API provides a JavaScript interface for accessing and
manipulating parts of the HTTP pipeline, such as requests and
responses.
https://youtu.be/drK6mdA9d_M

https://developer.mozilla.org/enUS/docs/Web/API/Fetch_API/Using_Fetch

6.Import / Export:
ES6 gave people ability to share code between their own
JavaScript files as well as third-party libraries using ES
modules.
https://youtu.be/s9kNndJLOjg

https://javascript.info/import-export

7.Async Js, Promise, Callback:
Such topics have been given in this thread, checkout.
https://youtu.be/ZYb_ZU8LNxs

8.Array functions- Map, Reduce, and Filter:

https://youtu.be/zdp0zrpKzIE

https://www.geeksforgeeks.org/how-to-map-reduce-and-filter-a-set-elementusingjavascript/#:~:text=The%20map()%2C%20reduce(),instead%20of%20using%20the%20loops.

If You Want to learn Javascript in detail you can refer
"Namaste Javascript" YouTube playlist 👇

https://youtube.com/playlist?list=PLlasXeu85E9cQ32gLCvAvr9vNaUccPVNP