class App extends React.Component{

   constructor(){

      super();

      this.state = {value: ''};

      this.onChange = this.onChange.bind(this)

   }

   onChange(e){

      const re = /^[0-9\b]+$/;

      if (e.target.value === '' || re.test(e.target.value)) {

         this.setState({value: e.target.value})

      }

   }

   render(){

     return <input value={this.state.value} onChange={this.onChange}/>

   }

}

​

ReactDOM.render(<App/>,document.getElementById('app'))

const onChanged = (text) => {

        let newText = '';

        let numbers = '0123456789';

        for (var i=0; i < text.length; i++) {

            if(numbers.indexOf(text[i]) > -1 ) {

                newText = newText + text[i];

            }

            else {

                alert("please enter numbers only");

            }

        }

        setNumber(newText);

    }