_el-add-class.js

/**
 * This method is several years old and is being produced here to continue the discussion 
 * about what is considered production-level element attribute toggling. 
 *
 * Both jQuery and twbs are widely used in production sites and can be used
 * as standards for production-level acceptance criteria. 
 *
 * jQuery is similar to this method - it looks for 
 * classList first and then falls back to className. 
 *
 * twbs4 only uses the classList object in components,
 * possibly because classList has been a standard for several years.
 *
 */

( function( _elements ) {
	
     /**
      * 
      * @param {Element}     element
      * @param {String}      className
      */
      var addClass = function( element, className ) {
  
            if ( element.nodeType !== 1 ) {
                throw new Error('Invalid Argument Type');
            }

	    if ( element.classList && element.classList.add ) {
	        element.classList.add( className );
	    } else if ( element.className ) {
	        element.className = element.className + ' ' + className;
	    }

	    return element;

	};


})( _elements );