ReactClass.js

import React from “react”;

class MyClass extends React.Component {
   constructor(props) {
       this.state = {
           count: 0,
           error: null
       }
       super(props);
   }
   validate() {
      let error = null;
      if(this.state.count > 10) {
         error = "count is too big";
      }
      if(this.state.count < 0) {
         error = "count can't be less than 0";
      }
      this.setState({
         error
      })
   }
   increaseCount() {
      this.setState({
         count: this.state.count + 1;
      })
      this.validate();
   }
   decreaseCount() {
      this.setState({
         count: this.state.count - 1;
      })
      this.validate();
   }
   render() {
       return (<div>
           <span>{`Count is: ${this.state.count}`}</span>
           <button
               onClick={
                   this.increaseCount();
               }>
            +
            </button>
            <button
               onClick={
                  this.decreaseCount();
               }
            >
            -
            </button>
            {this.state.error ?
               <div>{this.state.error}</div>
            : null}
       </div>)
   }
}