document.onload and getElementById( ) don't cooperate

[Erwin Moller]

Hi group,

I stumbled on something strange.

I simplified the problem to this:

A straightforward page with some JS:

<span id="testid">
Hi
</span>

<script type="text/javascript">
function alertID(){
var someID = document.getEle mentById("testi d");
alert (someID);
}

// call it after loading
document.onload = alertID();
</script>

will produce an alert saying [object]
as expected.
But if I 'program' the function before the span, like this, it produces
null.

<script type="text/javascript">
function alertID(){
var someID = document.getEle mentById("testi d");
alert (someID);
}
// call it after loading
document.onload = alertID();
</script>

<span id="testid">
Hi
</span>
That came as a surprise to me because I was thinking (and hoping) the
function would be evaluated AFTER the page loads.
It seems that JS is trying the function, then NOT finds the div (because it
is not on the page yet), and gives me null back, instead of evaluating the
function AFTER loading the page.

I have this behaviour on IE6 and Firefox.

Can anybody shed some light on this?

Regards,
Erwin Moller

[Erwin Moller]

Michael Winter wrote:

On 05/01/2006 16:15, Erwin Moller wrote:

[snip]

defer delays execution of the script untill body content is parsed and
rendered.
No, it doesn't. The defer attribute /hints/ that execution can be
delayed. It makes no promises about when, or even that a delay will occur.

Though waiting until the markup had been parsed would be sensible,
waiting until rendering has been performed is probably not.

Ok, don't blame me, I was just repeating what I read here:
http://www.websiteoptimization.com/speed/tweak/defer/

<quote>
First introduced by Internet Explorer 4, the defer attribute of the script
element is now part of the HTML 4 and XHTML specifications. The defer
attribute gives a hint to the browser that the script does not create any
content so the browser can optionally defer interpreting the script. This
can improve performance by delaying execution of scripts until after the
body content is parsed and rendered. Here's the brief paragraph describing
the defer attribute from the HTML 4.01 specification:

When set, this boolean attribute provides a hint to the user agent that
the script is not going to generate any document content (e.g., no
"document.write " in javascript) and thus, the user agent can continue
parsing and rendering.
</quote>

You are probably both right: they use the word 'may' instead of 'will',
making it all sound a lot less reliable.

Anyway, thanks for the warning.

Regards,
Erwin Moller

Mike

[Randy Webb]

Erwin Moller said the following on 1/5/2006 11:32 AM:

Michael Winter wrote:

On 05/01/2006 16:15, Erwin Moller wrote:

[snip]

defer delays execution of the script untill body content is parsed and
rendered.

No, it doesn't. The defer attribute /hints/ that execution can be
delayed. It makes no promises about when, or even that a delay will occur.

Though waiting until the markup had been parsed would be sensible,
waiting until rendering has been performed is probably not.

Ok, don't blame me, I was just repeating what I read here:
http://www.websiteoptimization.com/speed/tweak/defer/

<quote>
First introduced by Internet Explorer 4, the defer attribute of the script
element is now part of the HTML 4 and XHTML specifications. The defer
attribute gives a hint to the browser

"gives a hint to the browser".......
--
Randy
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