ReactJS - detect when the vertical scroll appears

Question

I would like to do some action when a vertical window's browser scroll appears. like this image below:

https://www.dropbox.com/s/t8th7cp7rcr662a/scroll.jpg?dl=0

This code below didn't work.

class App extends Component {
      constructor(props) {
        super(props)      
        this.onScroll = this.onScroll.bind(this)
      }

      onScroll(){
        window.onscroll = function (Event) {
          alert('the scroll is visible!')
        }
      }

      componentDidMount() {      
        window.addEventListener('onscroll', this.onScroll)
      }

      componentWillUnmount() {         
        window.removeEventListener('onscroll', this.onScroll)
      }

      render() {
        return (
          <div>
            <canvas id="canvas"></canvas>
            <div className="wrapper-all">
              <Coluna1 />
              <Coluna2 />
              <FooterMobile />
            </div>

          </div>
        )
      }
    }


    export default App;