It takes time to learn JavaScript thoroughly, and React as well. We learners should spend a good amount of time playing around with JavaScript language and the DOM in advanced.

Here is the list of JS concepts (with the links to reliable resources) to practice before you enter into the ReactJS world, so that you don't get overwhelmed with ReactJS.

• arrow functions
• variables and scopes
• difference between statements and expressions
• ES6 template literals
• rest and spread operators in arrays and objects
• array and objects destructuring
• mutable and immutable operations in JavaScript
• find and findIndex in arrays
• removing items from arrays with slice() and splice()
• array concat() and spread operator
• array map() and its usage
• using filter() on arrays to remove elements
• array reduce()
• importing and export modules and files
• conditionals and ternary operator

You can learn all of these "visually" for free by joining this Facebook group by CreaThit.