Handle the scroll event in react js and change the navbar class when special section is on top of windows

React Scroll Event

var Hello = React.createClass({

    getOffset : function(element){
      var bounding = element.getBoundingClientRect();
      return {
          top: bounding.top + document.body.scrollTop,
          left: bounding.left + document.body.scrollLeft
        };
    },

    handleScroll : function(){
       navbar = React.findDOMNode(this.refs.navbar);
       startElement = React.findDOMNode(this.refs.sec2);
       offset = this.getOffset(startElement)    
       windowsScrollTop  = window.pageYOffset;
       if(windowsScrollTop >= offset.top){     
         navbar.classList.add("navbar-fixed-top");           
       }else{
         navbar.classList.remove("navbar-fixed-top");   
       }
    },
    
    componentDidMount: function() {
        window.addEventListener('scroll', this.handleScroll);        
    },

    componentWillUnmount: function() {
    window.removeEventListener('scroll', this.handleScroll);
    },
    
    render: function() {
        return <div>
        <div className="navbar navbar-default" ref="navbar" id="navbar" role="navigation">
      <div className="container">
        <div className="navbar-header">
          <a className="navbar-brand" href="#">Project name</a>
        </div>        
      </div>
    </div>
            <section id="sec1"></section>
            <section id="sec2" ref="sec2"></section>
            <section id="sec3"></section>
        </div>;
    }
});
 
React.render(<Hello/>, document.getElementById('container'));

I also just made one. I created a Gist for it because it took me some time to figure out:

https://gist.github.com/agm1984/092ce379180e94bd13301bb33dbcd29f

Mine checks window.scrollY (debounced to every 16ms) and if the number is ever greater than 0,
it applies different CSS.

import React, { Component } from 'react'
import UserDetails from './UserDetails'

/**
 * This utility function allows function calls to be debounced.
 * @param {Function} func Function that requires debouncing
 * @param {Number} wait Wait time in milliseconds between successive invocations
 */
const debounce = (func, wait) => {
  let timeout
  return (...args) => {
    clearTimeout(timeout)
    timeout = setTimeout(() => func.apply(this, args), wait)
  }
}

class NavContainer extends Component {
  constructor(props) {
    super(props)
    this.state = {
      scrollPositionY: 0,
    }
  }

  componentDidMount() {
    // 32 is the number of milliseconds to debounce
    // I picked this because it's approx 1 frame (ie: 16.7ms)
    // You'll want to modulate that to your taste.
    // Add console.logs in handleScroll function to check if its flooding.
    return window.addEventListener('scroll', debounce(this.handleScroll, 16))
  }

  componentWillUnmount() {
    return window.removeEventListener('scroll', debounce(this.handleScroll, 16))
  }

  handleScroll = () => {
    // + is unary operator, same as Number(window.scrollY)
    const scrollPositionY = +window.scrollY
    return this.setState({ scrollPositionY })
  }

  render() {
    // !! coerces value to be a Boolean
    // we want it to be true or false (true if scrollPositionY> 0)
    // it works because scrollPositionY=== 0 is falsy
    const isScrolling = !!this.state.scrollPositionY
    return (
      <div className={(isScrolling) ? 'nav isScrolling' : 'nav'}>
        <UserDetails isScrolling={isScrolling} />
      </div>
    )
  }
}

NOTE: I encountered issues destructuring scrollY off window. That's why it's not.

If you make any state update while scrolling you will face choppy scroll performance issues