High performance utility for manipulating DOM classes.

dom-util.js

var testDiv = document.querySelector('.test-div');

function filterSpaces(strArray) {
  return strArray.filter(function(str) {
     return /\S/.test(str);
  });
}

function addClass(elem, className) {
  var classes = filterSpaces(elem.className.split(' '));
  if (className.constructor === Array) {
      var len = className.length;
      var i = 0;
      while (len--) {        
        console.log(i);
        classes.push(className[i]);
        i++;
      }
  } else {
    classes.push(className);
  }
  elem.className = classes.join(' ');
  return elem;
} 

function hasClass(elem, className) {
  var classes = filterSpaces(elem.className.split(' ')); 
  
  return classes.indexOf(className);
}

function toggleClass(elem, className) {
  var classes = filterSpaces(elem.className.split(' ')); 
  
  return (hasClass(elem, className) !== -1) ? removeClass(elem, className) : addClass(elem, className); 
}

function removeClass(elem, className) {
  var classes = filterSpaces(elem.className.split(' '));
  var index = classes.indexOf(className);
  
  if (index !== -1) {
      classes.splice(index, 1);
  }
  
  elem.className = classes.join(' ');
  
  return elem;
}
 
var testForClass = hasClass(testDiv, 'class-one');
console.log("test for hasClass: " + testForClass);

var testForAddClass = addClass(testDiv, 'class');  
console.log("test for addClass: " + testForAddClass.className);

var testForMultipleAddClass = addClass(testDiv, ['class-added', 'class-added-two']);  
console.log("test for multiple addClass: " + testForMultipleAddClass.className);

var testForRemoveClass = removeClass(testDiv, 'class-one');
console.log("test for removeClass: " + testForRemoveClass.className);

var testForToggleClass = toggleClass(testDiv, 'class-added');
console.log("test for toggleClass: " + testForToggleClass.className);

example here: http://jsbin.com/xijona/edit?html,js,output

inspect the element with chrome inspector when making changes to see classes being manipulated.