function moveElement()函数

function moveElement(elementID,final_x,final_y,interval) {
if (!document.getElementById) return false;
if (!document.getElementById(elementID)) return false;
var elem = document.getElementById(elementID);
var xpos = parseInt(elem.style.left);
var ypos = parseInt(elem.style.top);
if (xpos == final_x && ypos == final_y) {
return true;
}
if (xpos < final_x) {
xpos++;
}
if (xpos > final_x) {
xpos--;
}
if (ypos < final_y) {
ypos++;
}
if (ypos > final_y) {
ypos--;
}
elem.style.left = xpos + "px";
elem.style.top = ypos + "px";
var repeat = "moveElement('"+elementID+"',"+final_x+","+final_y+","+interval+")";
movement = setTimeout(repeat,interval);
}

请问各位老师，在此函数中var repeat = "moveElement('"+elementID+"',"+final_x+","+final_y+","+interval+")"; 为什么要拼接成一个字符串，不能写成这样
var repeat = "moveElement("elementID",final_x,final_y,interval)";
初学JavaScript，此处不理解，请各位老师指教，多谢！