index.js

import React from 'react';
import ReactDOM from 'react-dom';

class Welcome extends React.Component {
  constructor(props) {
    super(props);
    this.handleChange = this.handleChange.bind(this);
    this.state = {name: 'Medium'};
  }
  handleChange(e) {
    this.setState({name: e.target.value});
  }
  render() {
    return (
      <div style={{textAlign: 'center'}}>
        <h1>Welcome</h1>
        <p>Hello {this.state.name}</p>
        <input onChange={this.handleChange} defaultValue={this.state.name}/>
      </div>
    );
  }
}

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