the code is as follows
function moveElement(element,final_x,final_y,interval) {
if (!document.getElementById) return false;
if (!document.getElementById(element)) return false;
var elem = document.getElementById(element);
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);
if (xpos == final_x && ypos == final_y) {
return true;
}
if (xpos < final_x) {
var dist = Math.ceil((final_x - xpos)/10);
xpos = xpos + dist;
}
if (xpos > final_x) {
var dist = Math.ceil((xpos - final_x)/10);
xpos = xpos - dist;
}
if (ypos < final_y) {
var dist = Math.ceil((final_y - ypos)/10);
ypos = ypos + 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(""+element+"","+final_x+","+final_y+","+interval+")";
elem.movement = setTimeout(repeat,interval);
} this code is extracted from a variable elem, in Chapter 10
of the Art of Javascript DOM programming Edition 2. If I change it to get the node through the class name or item 0 in the tag signature,
for example
var elem = document.getElementsByTagName(element)[0];
var elem = document.getElementsByClassName(element)[0];in this way, the running function will report an error, and only by getting the node through id can it work normally. I really don"t understand
.what is the principle of this, dear gods? Thank you