Issue with Form validation

Hi fellow programmers, I kinda run into a bit of a jam for 1-1/2 days now with this form validation. Something is wrong with my if statement in the Function validateForm(form). If I take out the if statement, my alert is displayed on the screen. If I add the if statement, my alert does not display on the screen. Anyone see anything wrong with the code in the function as well as in the input/form and how the TSize name is reference in the function via the if statement? I think I have narrowed it down to the if statement but I have no clue if I am referencing the value of TSize correctly in the if statement. Thanks in advance for any helps.

<!---this form sets T value--->

<cfif not IsDefined("TSize")><cfset TSize = ""></cfif>

<script language="JavaScript"><!--
		
function validateForm(form){
	if (form.document.breast6.TSize.value =="") {
   	alert("Please enter a size.");
   	return false;
	} else {
		return true;
  }
}

// --></script>

<cfset diseasesite = "#site#">
<cfinclude template=stdheader.cfm>

<tr>
	<td colspan=2 bgcolor=ffFFff height=300 valign=top>&nbsp;</td>
	<td colspan=2 bgcolor=ffFFff>
	<cfoutput>
	<font size=2 face="Arial, Helvetica">
	<form name="breast6" method="post" action="breast6.cfm" onSubmit="return validateForm(this)">
	<b>Tumor Size:  </b><input type="text" name="TSize" size="10" value="">
	<P>
	(Use 00.0 if no evidence of primary tumor or 99.9 if size cannot be assessed)
	<input type="hidden" name="site" value="#site#">
	<input type="hidden" name="type" value="#type#">
	<input type="hidden" name="m" value="#m#">
	<input type="hidden" name="n" value="#n#">
	<input type="hidden" name="t" value="Unknown">
	<input type="hidden" name="TSize" value="#TSize#">
	<table>
	<tr><td></td></tr>
	<tr>
	<td align=left></td>
	<td>
	<input type="submit" value="Continue ...">
	</td>
	</tr>
	</table>
	</form>
	</font>
	</cfoutput>
	</td>
	<td bgcolor=ffFFff>&nbsp;</td>
</tr>

<cfinclude template=stdfooter.cfm>

Code (markup):

If I change the above function validateForm(form) to

function validateForm(form){
   	alert("Please enter a size.");
   	return false;
}

Code (markup):

The alert appears on the screen.

 

Give that form an ID.

And use this:

document.getElementById('id of element').value

Code (markup):

Then eval on that. (IE: == "")

 

Sorry, I don't quite understand the use of id. I have looked at www.w3schools.com/html but it is still not clear how I would use id in this cenario.

Edit: I am still a little haze on why I need the id. But I got it to work. Thanks to LittleJon helpful advice!
Here is what I changed in the code.

function validateForm(form){
var TSvalue=document.getElementById("yyy").value;
if (TSvalue =="") {
alert("Please enter a size.");
return false;
} else {
return true;
}
}



<b>Tumor Size: </b><input type="text" id="yyy" name="TSize" size="10" value="">
<P>

 

This way, is "standards":

<script type="text/javascript">
		
function validateForm(nForm){

	if (nForm['TSize'].value == "") 
		{
   		 alert("Please enter a size.");
		 return false;
		}
	return true;  
}

</script>

Code (markup):

 

Thanks Mike, will try that too. Everyone been very helpful!