How to add class in element on scroll React js

If you want to use React Hooks in 2020 stateless component

const [scroll, setScroll] = useState(false);
 useEffect(() => {
   window.addEventListener("scroll", () => {
     setScroll(window.scrollY > 50);
   });
 }, []);

and use it anywhere in your code

className={scroll ? "bg-black" : "bg-white"}

setScroll(window.scrollY > 50); here 50 specify the height.

Use state to manage classnames and update state in the scroll event. Also, you should move the scroll event binding into the componentDidMount function instead of render.

import React from "react"
import { Link } from "react-router"
import { prefixLink } from "gatsby-helpers"
import Helmet from "react-helmet"
import { config } from "config"

module.exports = React.createClass({
  propTypes() {
    return {
      children: React.PropTypes.any,
    }
  },
  componentDidMount(){
      window.addEventListener('scroll', () => {
         let activeClass = 'normal';
         if(window.scrollY === 0){
             activeClass = 'top';
         }
         this.setState({ activeClass });
      });
  }
  render() {
    return (
      <div>
        <header className="header">
          <div className={`top-bar ${this.state.activeClass}`}>
            <span><a href="tel:6788272782">678-827-2782 </a></span>
            <span><a href="mailto:[email protected]"> [email protected]</a></span>
            <button>Login</button>
          </div>
        </header>
        {this.props.children}
      </div>
    )
  },

})

One more variant with hooks and cleanup to avoid memory leaks, if the browser is not handled it properly.

const [isScrollValueMoreThanHeaderHeight, setIsScrollValueMoreThanHeaderHeight] = useState(false);

//here 96(px) - height of current header

useEffect(() => {
    const handleScroll = () => {
        setIsScrollValueMoreThanHeaderHeight(window.scrollY > 96);
    }
    window.addEventListener('scroll', handleScroll)
    return () => window.removeEventListener('scroll', handleScroll);
},[])

And then used it in a ternary way to change class

className={isScrollValueMoreThanHeaderHeight ? "class1" : "class2"}