I need to check a ASP variable value in a Javascript function , how do i do this?

Very easy, I do it all the time.

If you're using a form, just put the ASP variable into a hidden field and then have javascript check the value in the hidden field.

<form name="formName" id="formName">
<input type="hidden" name="varASP" id="varASP" value="<%= the.ASPvariable %>">
</form>

Then just have your Javascript read it normally:

var checkMe = document.forms['formName'].varASP.value;

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A shorter way is to just skip the form (but it's useful to mention that first because most of the times I need to check an ASP variable it's usually for a form) and reference the ASP variable directly in the javascript:

var checkMe = "<%= the.ASPvariable %>";

Note the quotes around the ASP code. That's pretty important.

Good luck!

And for the same kind of answer in other words (that hopefully might give you some more ideas along the same lines):

It's easy to use the value of a VBScript variable in JavaScript:
document.writeln('<%= MyVBScriptVariable %>');

It's also easy to use the value of a VBScript function in javascript:
document.writeln('<%= MyVBScriptFunction() %>');

Or here is an example which puts the value of a VBScript variable into a JavaScript alert box:
Response.Write "<script language='JavaScript'>alert('The value is " & MyVBScriptVariable & "');</script>"