表单自动tab小函数

    addEvent(fm.elements['a1'], 'keyup', tabForWard);
    addEvent(fm.elements['a2'], 'keyup', tabForWard);
    addEvent(fm.elements['a3'], 'keyup', tabForWard);

    function tabForWard(evt){
        var e = evt || window.event;
        if(this.value.length == this.maxLength){
            for(var i=0; i<fm.elements.length; i++){
                if(fm.elements[i] == this){
                    fm.elements[i+1].focus();
                    return;
                }
            }
        }
    }