Vanilla JS function to add class on DOM elements.

AddClass.js

function addClass(elements, myClass) {

  // if there are no elements, we're done
  if (!elements) { return; }

  // if we have a selector, get the chosen elements
  if (typeof(elements) === 'string') {
    elements = document.querySelectorAll(elements);
  }

  // if we have a single DOM element, make it an array to simplify behavior
  else if (elements.tagName) { elements=[elements]; }

  // add class to all chosen elements
  for (var i=0; i<elements.length; i++) {

    // if class is not already found
    if ( (' '+elements[i].className+' ').indexOf(' '+myClass+' ') < 0 ) {

      // add class
      elements[i].className += ' ' + myClass;
    }
  }
}