States

constructor(props) {
  super(props)
  this.state = { username: undefined }
}

this.setState({ username: 'winplaybox' })

render () {
  this.state.username
  const { username } = this.state
  ···
}

Use states (this.state) to manage dynamic data.

With Babel you can use proposal-class-fields and get rid of constructor

class Hello extends Component {
  state = { username: undefined };
  ...
}

See: States