Remove CSS class from element with JavaScript (no jQuery)

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');
  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|$)'), ' ');

Try this:

function hasClass(ele, cls) {
  return ele.className.match(new RegExp('(\\s|^)'+cls+'(\\s|$)'));
}
    
function removeClass(ele, cls) {
  if (hasClass(ele,cls)) {
    var reg = new RegExp('(\\s|^)'+cls+'(\\s|$)');
    ele.className=ele.className.replace(reg,' ');
  }
}

try:

function removeClassName(elem, name){
    var remClass = elem.className;
    var re = new RegExp('(^| )' + name + '( |$)');
    remClass = remClass.replace(re, '$1');
    remClass = remClass.replace(/ $/, '');
    elem.className = remClass;
}

var element = document.getElementById('example_id');
var remove_class = 'example_class';

element.className = element.className.replace(' ' + remove_class, '').replace(remove_class, '');