Remove CSS class from element with JavaScript (no jQuery) [duplicate]

Could anyone let me know how to remove a class on an element using JavaScript only?
Please do not give me an answer with jQuery as I can’t use it, and I don’t know anything about it.

The right and standard way to do it is using classList. It is now widely supported in the latest version of most modern browsers:

ELEMENT.classList.remove("CLASS_NAME");

remove.onclick = () => {
  const el = document.querySelector('#el');
  if (el.classList.contains("red")) {
    el.classList.remove("red");

  }
}

.red {
  background: red
}

<div id='el' class="red"> Test</div>
<button id='remove'>Remove Class</button>

Documentation: https://developer.mozilla.org/en/DOM/element.classList