Changing the CSS of a div with classname using JavaScript

Question

I want to change the animation attribute of my CSS class with a JavaScript function. I have been trying various methods but none of them worked. Please find out where I am going wrong.

CSS:

   .center {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 200px;
  height: 200px;
  border-radius: 50%;
  background: transparent;
}

.center .earth {
  position: absolute;
  top: 0;
  width: 200px;
  height: 200px;
  background-image: url(https://www.publicdomainpictures.net/pictures/90000/velka/earth-map.jpg);
  margin: 3em auto;
  border-radius: 50%;
  background-size: 630px;
  animation: spin 30s linear alternate infinite;
  box-shadow: inset 20px 0 80px 6px rgba(0, 0, 0, 1);
  color: #000;
}

.center .earth .moon {
  position: absolute;
  top: calc(50% - 1px);
  left: 50%;
  width: 200px;
  height: 2px;
  transform-origin: left;
  border-radius: 50%;
  animation: rotate 10s linear infinite;
}

.center .earth .moon::before {
  content: '';
  background-image: url(https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcRsvjrANMGI8aBJSFbsHteVa04rcB1IjjNsbrhm8vTLflfpiG133g);
  position: absolute;
  background-size: 200px;
  top: -25px;
  right: 0;
  width: 50px;
  height: 50px;
  border-radius: 50%;
  animation: spin 10s linear infinite;
}

@keyframes rotate {
  0% {
    transform: rotate(0deg);
  }
  100% {
    transform: rotate(360deg);
  }
}

@keyframes spin {
  100% {
    background-position: 100%;
  }
}

HTML:

<div className="center">
      <div className="earth">
        <div className="moon" />
      </div>
    </div>

JavaScript function:

    function change() {
  var elem = document.getElementsByClassName('moon');
  elem.style.animation="rotate 5s linear infinite";
}

I have tried various methods but none of them are able to perform the modification. Looping through the object didn't help either.

Answer 1

elem is an HTMLCollection of elements, not a single element, and setting the style property of the HTMLCollection doesn't work.

Instead, you'll want to change the style for each element individually:

Array.from(document.getElementsByClassName('moon')).forEach(elem => {
    elem.style.animationDuration = "5s";
});

EDIT:

Here's a snippet to show that the method works/provide a reference:

function change_spin_rate() {
  Array.from(document.getElementsByClassName('spinner')).forEach(elem => {
    elem.style.animationDuration = "1s";
  });
}

document.getElementById("spin_change").onclick = change_spin_rate;

@keyframes spin {
  from {
    transform: rotate(360deg);
  }
  to {
    transform: rotate(0deg);
  }
}

.spinner {
  display: inline-block;
  animation: spin 4s infinite linear;
}

#spin_change {
  padding: 8px 16px;
  display: inline-block;
  font-family: sans-serif;
  cursor: pointer;
  border-radius: 16px;
}

#spin_change:hover {
  background: rgba(0, 64, 255, 0.1);
}

<a class="spinner">🌀</a><a class="spinner">🌀</a><a class="spinner">🌀</a>

<a id="spin_change">Click me to change the spin rate for all spinners.<a>

<a class="spinner">🌀</a><a class="spinner">🌀</a><a class="spinner">🌀</a>

Answer 2

document.getElementsByClassName returns multiple elements. You need to iterate through them and apply your change to each element.

function change() {
  Array.from(document.getElementsByClassName("moon")).forEach((elem) => {
    elem.style.animation = "rotate 5s linear infinite";
  });
}

document.getElementsByClassName doesn't return an array. You have to use Array.from on it before you can call Array.prototype.forEach.

By the way, <div className="moon" /> should be <div class="moon" /> unless you are writing React JSX.

Answer 3

try this:

function change() {
  let elems = document.getElementsByClassName("moon");
  elems.forEach(elem => elem.style.animation = "rotate 5s linear infinite");
}