There is no hard and fast rule to start learning React. However, basic programming knowledge, preferably in JavaScript is a must to understand the concepts that we are going to apply and learn.
Here are some of the concepts which I feel one should look into as per my experience:
let, var and const: We should know what is the difference between them and where to use them. Along with that, one should have solid fundamentals of data types.
Conditional and Loops: As I already mentioned, one should know how to use loops, especially the "ForEach" loop.
Functions and Arrow Functions: How to write and call functions and Arrow Functions. What is the difference between Functions & Arrow functions? What is lexical in Arrow Function.
Event Handling: Event handlers can be used to handle and verify user input, user actions, and browser actions.
Promises & Async/Await: To understand the asynchronous nature of JavaScript, we should have understood these concepts very well.
map()/filter()/reduce(): Map, reduce, and filter are all array methods in JavaScript. Each one will iterate over an array and perform a transformation or computation. Each will return a new array based on the result of the function.
Module exports and imports
Again, I would say there is no hard and fast rule to start learning React. React is a library that provides ways to easily write the code and develop. But, one should at least understand these topics to not get overwhelmed.