index.js
index.html
 


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

class MyForm extends React.Component {
  constructor(props) {
    super(props);
    this.state = { username: '' };
  }
  myChangeHandler = (event) => {
    this.setState({username: event.target.value});
  }
  render() {
    let header = '';
    if (this.state.username) {
      header = <h1>Hello {this.state.username}</h1>;
    } else {
      header = '';
    }
    return (
      <form>
      {header}
      <p>Enter your name:</p>
      <input
        type='text'
        onChange={this.myChangeHandler}
      />
      </form>
    );
  }
}

ReactDOM.render(<MyForm />, document.getElementById('root'));
localhost:3000