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Toggle Class based on scroll React JS

I’m using bootstrap 4 nav bar and would like to change the background color after ig 400px down scroll down. I was looking at the react docs and found a onScroll but couldn’t find that much info on it. So far I have…

I don’t know if I’m using the right event listener or how to set the height etc.

And I’m not really setting inline styles… 

  import React, { Component } from 'react';

   class App extends Component {

   constructor(props) {
    super(props);

      this.state = {  scrollBackground: 'nav-bg' };
      this.handleScroll = this.handleScroll.bind(this);
   }


   handleScroll(){
      this.setState ({
         scrollBackground: !this.state.scrollBackground
       })
    }

 render() {
 const scrollBg = this.scrollBackground ? 'nav-bg scrolling' : 'nav-bg';

 return (
   <div>

       <Navbar inverse toggleable className={this.state.scrollBackground} 
                                  onScroll={this.handleScroll}>
        ...
      </Navbar>

    </div>
   );
  }
}

export default App;

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Answer

One way to add a scroll listener is to use the componentDidMount() lifecycle method. Following example should give you an idea:

import React from 'react';
import { render } from 'react-dom';

class App extends React.Component {
  state = {
    isTop: true,
  };

  componentDidMount() {
    document.addEventListener('scroll', () => {
      const isTop = window.scrollY < 100;
      if (isTop !== this.state.isTop) {
          this.setState({ isTop })
      }
    });
  }
  render() {
    return (
      <div style={{ height: '200vh' }}>
        <h2 style={{ position: 'fixed', top: 0 }}>Scroll {this.state.isTop ? 'down' : 'up'}!</h2>
      </div>
    );
  }
} 

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

This changes the Text from “Scroll down” to “Scroll up” when your scrollY position is at 100 and above.

Edit: Should avoid the overkill of updating the state on each scroll. Only update it when the boolean value changes.

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