JAVASCRIPT

what is react mounting

React has four built-in methods that gets called, 
  in this order, when mounting a component:

constructor()
getDerivedStateFromProps()
render()
componentDidMount()
The render() method is required and will always be called, the 
others are optional and will be called if you define them.

mounting in react

class App extends React.Component {
  state = {
    showUser: false
  }

  render() {
    return (
      <div>
        {this.state.showUser && <User name="Brad" />}
        <button onClick={() => this.setState({ showUser: true })}>
          Show User
        </button>
        <button onClick={() => this.setState({ showUser: false })}>
          Hide User
        </button>
      </div>
    )
  }
}

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