Should I learn ES6 before learning ReactJS?

No, you don’t have to learn es2015/es6 to learn react. But you may find many resources or React ecosystem uses ES6 extensively. Let's see some of the frequently used ES6 features,

1. Destructuring: To get props and use them in a component

   // in es 5
   var someData = this.props.someData;
   var dispatch = this.props.dispatch;
   
   // in es6
   const { someData, dispatch } = this.props;

2. Spread operator: Helps in passing props down into a component

   // in es 5
   <SomeComponent someData={this.props.someData} dispatch={this.props.dispatch} />
   
   // in es6
   <SomeComponent {...this.props} />

3. Arrow functions: Makes compact syntax

   // es 5
   var users = usersList.map(function (user) {
     return <li>{user.name}</li>;
   });
   // es 6
   const users = usersList.map((user) => <li>{user.name}</li>);