Question

I would like the ability to add a class to an element and not replace what classes are already there.

Here is my javascript so far:

function black(body) {
var item = document.getElementById(body);
    if (item) {
        item.className=(item.className=='normal')?'black':'normal';
    }
}

That piece of javascript replaces the existing classes with black. If the class already is black then it is changed to normal.

I would like to somehow combine the script above with the script below script, which adds a class, instead of replacing all existing classes.

var item = document.getElementById("body");
item.className = item.className + " additionalclass";

Answer 1

Here are several plain javascript functions you can use for manipulating class names in plain javascript. It takes a little extra work in these functions to match whole class names only and avoid any extra spaces before/after classnames:

function removeClass(elem, cls) {
    var str = " " + elem.className + " ";
    elem.className = str.replace(" " + cls + " ", " ").replace(/^\s+|\s+$/g, "");
}

function addClass(elem, cls) {
    elem.className += (" " + cls);
}

function hasClass(elem, cls) {
    var str = " " + elem.className + " ";
    var testCls = " " + cls + " ";
    return(str.indexOf(testCls) != -1) ;
}

function toggleClass(elem, cls) {
    if (hasClass(elem, cls)) {
        removeClass(elem, cls);
    } else {
        addClass(elem, cls);
    }
}

function toggleBetweenClasses(elem, cls1, cls2) {
    if (hasClass(elem, cls1)) {
        removeClass(elem, cls1);
        addClass(elem, cls2);
    } else if (hasClass(elem, cls2)) {
        removeClass(elem, cls2);
        addClass(elem, cls1);
    }
}

If you wanted to toggle between the black and normal classes without affecting any other classes on the specified object, you could do this:

function black(id) {
    var obj = document.getElementById(id);
    if (obj) {
        toggleBetweenClasses(obj, "black", "normal");
    }
}

Working example here: http://jsfiddle.net/jfriend00/eR85c/

If you wanted to add the “black” class unless “normal” was already present, you could do this:

function black(id) {
    var obj = document.getElementById(id);
    if (obj && !hasClass(obj, "normal")) {
        addClass(obj, "black");
    }
}