<script type="text/javascript">
function bx(bxname) {
    this.boxname = bxname;

    this.show = function() {
        var box = document.getElementById(this.bxname); //Funktioniert
        box.style.display = 'block';
        
        window.onmousemove = this.mm;
    }

    this.mm = function(event) {
        if(!event) {var event = window.event;}
        var box = document.getElementById(this.boxname); //Funktioniert nicht
    }
}
</script>

function tooltip(tooltipbox) {
    this.boxname = tooltipbox;
    
    this.show = function(text) {
        var box = document.getElementById(this.boxname);
        
        box.style.display = 'block';
        box.style.position = 'absolute';
        box.innerHTML = text;
        
        window.onmousemove = this.move;
    }
    
    this.move = function(event) {
        if(!event) {var event = window.event;}
        var box = document.getElementById(this.boxname);
        
        if(box.style.display != 'none') {
            box.style.top = event.clientY+(box.offsetHeight);
            box.style.left = event.clientX;
            if(box.offsetHeight+event.clientY > window.innerHeight) {
                box.style.top = window.innerHeight-box.offsetHeight;
            }
            if(box.offsetWidth+event.clientX > window.innerWidth) {
                box.style.left = window.innerWidth-box.offsetWidth;
            }
        }
    }
    
    this.hide = function() {
        var box = document.getElementById(this.boxname);
        box.style.display = 'none';
    }
}

if(!event) {var event = window.event;}

<script type="text/javascript">
var tooltip = new tooltip('tooltip');
</script>