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;
    }
})();

@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.