This function is used to create an animation effect:

function moveElement(elementId,final_x,final_y,interval){ // 名称 id x轴位置 y轴位置 速度 var elem=document.getElementById(elementId); var x_pos=parseInt(elem.style.left); var y_pos=parseInt(elem.style.top); if(elem.movement){ clearTimeout(elem.movement); } //若elem已存在一个movement属性，先结束它 if(y_pos==final_y&&x_pos==final_x){ return true; } if(y_posfinal_y){ dist=Math.ceil((y_pos-final_y)/30); y_pos=y_pos-dist; } if(x_posfinal_x){ dist=Math.ceil((x_pos-final_x)/30); x_pos=x_pos-dist; } elem.style.top=y_pos+"px"; elem.style.left=x_pos+"px"; repeat="moveElement('"+elementId+"',"+final_x+","+final_y+","+interval+")" elem.movement=setTimeout(repeat,interval) // 自定义的属性movement，它属于elem }

Regarding the last sentence elem.movement=setTimeout(repeat,interval)
and the above sentence if(elem.movement){

clearTimeout(elem.movement); }

I know that their function is to end the function that was run last time when running again, but why does it report an error when directly writing movement=setTimeout(repeat,interval), and can it be written as elem.movement?