How to enable/disable buttong based on session variable

if (Session["url"] != null)
	{
		ImportButton.disabled = false;
		var filename = Session["url"];
		Session.Contents.Remove(url)
	}
	else
	{
		ImportButton.disabled = true;
	}
	
    <form name="form1">
    Please choose a file.
		<input type="file" name="uploadbox" size="35" onChange='check_extension(this.value, "UploadButton")'>
		<br> <br>
		<input type="submit" id="UploadButton" value="Upload" disabled="disabled"/>
		<input type="submit" id="ImportButton" value="Import" disabled="disabled"/>
		<input type="submit" id="CloseButton"  value="Close"  onClick='window.close()'/>
    </form>
    </body>
    </html>

Above is just a sample code in html. i want to enable/disable button based on session variable URL. Also, is the clearing logic for session variable is correct?

JavaScript has no access to the server-side session. Where did you find this API that you are using?