Element.classList

const div = document.createElement('div');
div.className = 'foo';

// Status zum Beginn: <div class="foo"></div>
console.log(div.outerHTML);

// classList-API zum Entfernen und Ergänzen von Klassen nutzen
div.classList.remove("foo");
div.classList.add("anotherclass");

// <div class="anotherclass"></div>
console.log(div.outerHTML);

// Wenn visible gesetzt ist entferne es, sonst füge es hinzu
div.classList.toggle("visible");

// Hinzufügen/Enfernen von visible, abhängig von der Bedingung, ob i kleiner 10 ist
div.classList.toggle("visible", i < 10 );

console.log(div.classList.contains("foo"));

// Mehrere Klassen hinzufügen / entfernen
div.classList.add("foo", "bar", "baz");
div.classList.remove("foo", "bar", "baz");

// Mehrere Klassen mittels Spread-Syntax hinzufügen / entfernen
const cls = ["foo", "bar"];
div.classList.add(...cls);
div.classList.remove(...cls);

// Klasse "foo" durch "bar" ersetzen
div.classList.replace("foo", "bar");

// Source: https://gist.github.com/k-gun/c2ea7c49edf7b757fe9561ba37cb19ca
/**
 * Element.prototype.classList for IE8/9, Safari.
 * @author    Kerem Güneş <k-gun@mail.com>
 * @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;
    }
})();