DOM自学中的一个问题，求解答，谢谢

12345678910111213141516171819202122232425262728293031323334353637383940function moveElement(elementID,final_x,final_y,interval) {    if (!document.getElementById) return false;    if (!document.getElementById(elementID)) return false;    var elem = document.getElementById(elementID);    if (elem.movement) {        clearTimeout(elem.movement);    }    if (!elem.style.left) {        elem.style.left = "0px";    }    if (!elem.style.top) {        elem.style.top = "0px";    }    var xpos = parseInt(elem.style.left);    var ypos = parseInt(elem.style.top);    var dist = 0;    if (xpos == final_x     }    if (xpos final_x) {        var dist = Math.ceil((xpos - final_x)/10);        xpos = xpos - dist;    }    if (ypos final_y) {        var dist = Math.ceil((ypos - final_y)/10);        ypos = ypos - dist;    }    elem.style.left = xpos + "px";    elem.style.top = ypos + "px";    var repeat =         "moveElement('"+elementID+"',"+final_x+","+final_y+","+interval+")";    elem.movement = setTimeout(repeat,interval);    }

请问这行代码 var repeat = "moveElement('"+elementID+"',"+final_x+","+final_y+","+interval+")";为什么不直接写成 var repeat = moveElement("elementID",final_x,final_y,interval);为什么要用字符串的形式？谢谢~