我该如何改变颜色 <nav> 当我在手机上滚动时？

Question

I already did it on desktop, but when I try the code with mobile, it crashes for some reason

    window.onscroll = () => {
  const nav = document.querySelector('#navbar');
 var viewportWidth = $(window).width();        
if (viewportWidth > 1020) {
        if(this.scrollY <= 500) nav.className = ''; else nav.className = 'scroll';}

};

Answer 1

Responsive design tends to be really hard to emulate using JS, so don't do more than necessary with JS:

window.onscroll = () => {
  const nav = document.querySelector('#navbar');
  nav.className = (this.scrollY <= 500) ? '' : 'scroll';
};

And use CSS media queries to have the color only be changed on mobile:

#navbar.scroll {
  /* when scrolling on desktop */
  background-color: green;
}


@media screen and (max-width: 1019px) {
  #navbar.scroll {
    /* when scrolling on tablets or mobile */
    background-color: red;
  }
}