I think it's related to web server setting, don't know which forum should be posted.
When I try to access index.html file from browser's url address, I have to type: www.mydomain.com/folder/
if I type: www.mydomain.com/folder
then it shows "no page found", I want to know how to omit the last "/"? any idea?

I'm not sure I understand the problem. If you use the URL:

www.mydomain.com/folder

Then you're requesting a file named "folder" in the root folder of that domain. When you use the URL:

www.mydomain.com/folder/

Then you're asking for the index file, which can be various files depending on server configuration, but is usually "index.html", from the "folder" sub-folder of that domain.

This is normal, accurate behavior and doesn't indicate any error.

I suppose you could create a file named "folder" that did some sort of re-direct. Or create a virtual mapping in your server configuration. But why? You're seeing perfectly normal behavior. In any case, you should ask in the "Server Configuration" forum, as JavaScript programming cannot help you with this.