左按钮的问题

(function(){

    var carousel=document.getElementById('carousel')

    var rightBtn=document.getElementById('right-btn')

    var leftBtn=document.getElementById('left-btn')

    var kelong=carousel.firstElementChild.cloneNode(true)

    carousel.appendChild(kelong)

    var index=0

    var lock=true;

    rightBtn.onclick=function(){

        if(!lock) return

        lock=false;

        index++

        carousel.style.transition='transform 0.5s ease 0s'

        if(index>4){

            setTimeout(function(){

                carousel.style.transition='none'

                carousel.style.transform='none'

                index=0

            },500)

        }

        carousel.style.transform='translateX('+ -16.66*index +'%)'

        setTimeout(function(){

            lock=true;

        },500)

    }

    leftBtn.onclick=function(){

        if(!lock) return;

        lock=false;

        if(index==0){

            carousel.style.transition='none'

            carousel.style.transform='translateX('+ -16.66*5 +'%)'

            setTimeout(function(){

                carousel.style.transition='transform 0.5s ease 0s'

                carousel.style.transform='translateX('+ -16.66*4 +'%)'

                index=4;

            },0)

        }else{

            index--

            carousel.style.transform='translateX('+ -16.66*index +'%)'

        }

        setTimeout(function(){

            lock=true;

        },500)

    }

})();

请老师看下，为什么点左按钮轮回4下会突然拉回到最后一张图？？