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

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

document.getElementById("MyID").className =
    document.getElementById("MyID").className.replace(/\bMyClass\b/,'');

where MyID is the ID of the element and MyClass is the name of the class you wish to remove.

UPDATE: To support class names containing dash character, such as "My-Class", use

document.getElementById("MyID").className =
  document.getElementById("MyID").className
    .replace(new RegExp('(?:^|\\s)'+ 'My-Class' + '(?:\\s|$)'), ' ');