Polyfill: Element.prototype.classList for IE8/9, Safari.

Element.prototype.classList.js

/**
 * Element.prototype.classList for IE8/9, Safari.
 * @author    Kerem Güneş <[email protected]>
 * @copyright Released under the MIT License <https://opensource.org/licenses/MIT>
 * @version   1.2
 * @see       https://developer.mozilla.org/en-US/docs/Web/API/Element/classList
 */
;(function() {
    // Helpers.
    var trim = function(s) {
            return s.replace(/^\s+|\s+$/g, '');
        },
        regExp = function(name) {
            return new RegExp('(^|\\s+)'+ name +'(\\s+|$)');
        },
        forEach = function(list, fn, scope) {
            for (var i = 0; i < list.length; i++) {
                fn.call(scope, list[i]);
            }
        };

    // Class list object with basic methods.
    function ClassList(element) {
        this.element = element;
    }

    ClassList.prototype = {
        add: function() {
            forEach(arguments, function(name) {
                if (!this.contains(name)) {
                    this.element.className = trim(this.element.className +' '+ name);
                }
            }, this);
        },
        remove: function() {
            forEach(arguments, function(name) {
                this.element.className = trim(this.element.className.replace(regExp(name), ' '));
            }, this);
        },
        toggle: function(name) {
            return this.contains(name) ? (this.remove(name), false) : (this.add(name), true);
        },
        contains: function(name) {
            return regExp(name).test(this.element.className);
        },
        item: function(i) {
            return this.element.className.split(/\s+/)[i] || null;
        },
        // bonus
        replace: function(oldName, newName) {
            this.remove(oldName), this.add(newName);
        }
    };

    // IE8/9, Safari
    // Remove this if statements to override native classList.
    if (!('classList' in Element.prototype)) {
    // Use this if statement to override native classList that does not have for example replace() method.
    // See browser compatibility: https://developer.mozilla.org/en-US/docs/Web/API/Element/classList#Browser_compatibility.
    // if (!('classList' in Element.prototype) ||
    //     !('classList' in Element.prototype && Element.prototype.classList.replace)) {
        Object.defineProperty(Element.prototype, 'classList', {
            get: function() {
                return new ClassList(this);
            }
        });
    }

    // For others replace() support.
    if (window.DOMTokenList && !DOMTokenList.prototype.replace) {
        DOMTokenList.prototype.replace = ClassList.prototype.replace;
    }
})();

Also on line 28, for the remove(), if the class being removed is in the middle of two other classes rather than an end (and if classes are delimited by only a single space), it loses the space and the surrounding class names get combined into one.

@ballercat; even it is not worth to deal with empty class, fixed it.
@desjardinsm; that was a good catch, thank you, fixed.

And added item() and other trivial stuff, comments etc.