Full Code of State

import React, { Component } from 'react'

export class ReactDemo extends Component {
    constructor () {
        super()
        this.state = {
            username: ''
        }
    }

    loggingIn () {
        this.setState({
            username: 'joy'
        })
    }

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

    render () {
        <div>      <Login onClick={this.loggingIn} />
            <Logout onClick={this.loggingOut} />
        </div>
    }
}