Question

how to set element attribute with javascript in IE6..? It seems setAttribute doesn’t work. I really need to do it on the fly. Thanks.

Code

<script type="text/javascript" language="javascript"> 
    menuItems = document.getElementById("menu").childNodes; 
    for (i = 0; i < menuItems.length; i++)
    { 
        if (menuItems[i].className != "blue") 
            menuItems[i].setAttribute('onmouseover', 'HoverMenu(this)'); 
    } 
</script>

Answer 1

(Most of the below was before the OP clarified they were setting an event handler; left the list of other issues in case others find them useful)

IE6 makes a mess of several aspects of setAttribute. Without knowing the exact problem you were dealing with (this was before the edit inidicating it was an event handler), it’s hard to be sure whether that’s really the problem, but here are a couple of known issues:

You can’t use setAttribute to set event handlers

It’s best to set event handlers using the reflected property or with addEventListener [standard] / attachEvent [IE], not setAttribute (and you can’t use setAttribute on IE).

So, any of these will work:

// Using reflected property
theElement.onclick = yourFunction;

// Using DOM2 standard addEventListener; note it's "click", not "onclick"
theElement.addEventListener("click", yourFunction, false);

// IE's non-standard alternative to addEventListener
theElement.attachEvent("onclick", yourFunction);

…not

// This doesn't work on IE (at least)
theElement.setAttribute("onclick", "yourFunction();");

The addEventListener / attachEvent way of doing this is cool for other reasons: You can have multiple event listeners on the same event of an element. You can’t do that using the reflected property.

So for your specific situation:

menuItems = document.getElementById("menu").childNodes; 
for (i = 0; i < menuItems.length; i++)
{ 
    if (menuItems[i].className != "blue") {
        menuItems[i].onmouseover = function() {
            HoverMenu(this);
        }
    }
}

Certain attributes need their modified names

class

The correct way to set the class attribute is to use the reflected property className:

// Correct, cross-browser way. Works on IE6+, all versions of
// Chrome, etc. Completely reliable.
theElement.className = "new set of classes";

or on modern browsers (so, not IE6!) you can use classList.

One of IE6’s many setAttribute bugs is that this doesn’t work:

// Should also work, but fails on IE6 (and probably some other old versions)
theElement.setAttribute("class", "new set of classes");

Instead, IE6 (and probably a couple of other versions) make you use "className" instead of "class", even though that makes no sense whatsoever. The reflected property has the name className because it used to be that in JavaScript, you couldn’t use reserved words (like class or for or if) as property literals (obj.class was invalid). That’s not been true for a while now (ECMAScript 5 fixed it), but that’s why the reflected property is className, not class.

But since setAttribute takes a string, it should accept the proper name of the attribute. The fact it doesn’t is just an IE bug (and one they’ve fixed in modern versions of IE if they’re not in [in]compatibility mode).

for

Similarly, to set the for attribute (for instance, on labels), one uses the htmlFor reflected property:

// Correct, cross-browser way. Works on IE6+, all versions of
// Chrome, etc. Completely reliable.
theLabel.htmlFor = "id-of-field";

On any non-broken browser, you can also use setAttribute:

// Should also work, but fails on IE6 (and probably some other old versions)
theLabel.setAttribute("for", "id-of-field");

…but not on IE6, it wants "htmlFor" instead of "for" for the same reason it wants "className" rather than "class" (e.g, because it’s broken).

This page has quite a list of attributes that are problematic with IE.

setAttribute can’t be used to set the style attribute

…you have to use the style property instead; but to be fair, that’s usually a more convenient way. Example: This won’t work on IE:

theElement.setAttribute("style", "color: blue"); // Doesn't work on IE

…but this will:

myElement.style.color = "blue";

Slightly OT: Look at libraries

JavaScript libraries like Prototype, jQuery, Closure, or any of several others will make most of the above a lot easier and smooth out differences amongst browsers if you go through their APIs.