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) {        }   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); }

其中：var repeat = "moveElement(+elementID+,"+final_x+","+final_y+","+interval+")";的+elementID+前面的加号和+interval+后面的加号的作用是什么呢？