为什么加入if判断后，图片切不了了啊

Question

var list = document.getElementById('list');var prev = document.getElementById('prev');var next = document.getElementById('next');function animate (offset) {         var newLeft =  parseInt(list.style.left) + offset;         list.style.left = newLeft+'px';         if(newLeft>-1000){             list.style.left=–3000+'px';         }         if(newLeft<-3000){             list.style.left=–1000+'px';         }     }     next.onclick = function () {         animate(-1000);        }    prev.onclick = function () {        animate(1000);    }