When Event Run Function Not Working

In the file script.js I got 2 functions:
Code:
function tooltip(){
var m=document.getElementsByTagName("A");
var k=[];
var mover = document.createAttribute("onMouseOver");
mover.nodeValue = "showtooltip()";
for (var i = 0; i<m.length; i++) {
var mover = document.createAttribute("onMouseOver");
mover.nodeValue = "showtooltip()";
m[i].setAttributeNode(mover)
}}
This one is working. It's putting into <a href="#" title="some title" /> an attribute: onMouseOver with value: showtooltip()

And that's my 2nd function.
Code:
function showtooltip() {
document.write(this.getAttribute("title"));
}

When event is run the function isn't working. But when my html looks like this:
Code:
<a href="#" title="some title" onMouseOver="document.write(this.getAttribute("title"))">link</a>
Everything is fine. And javascript console doesn't show any errors.

My 2nd problem is, that onMouseOver doesn't work in chrome (I got dev version). Even function like this:
Code:
showtooltip() {
document.write("......");
}