To do a postback via JavaScript you can call the following server side to create the JavaScript code for you:

string postBackJavascript = Page.GetPostBackEventReference(yourControl);

This will return the __doPostBack JavaScript code as a string, and you will need place it on your page attached to something or you can call the __doPostBack directly on your own with:

__doPostBack(yourControlId,'');

If you're doing it yourself and not using Page.GetPostBackEventReference then make sure to get the ClientID for the control that triggered the validation, like:

__doPostBack('<%= yourControl.ClientID %>','');

EDIT: After re-reading your question you didn't say you wanted to trigger the postback based on an ASP.NET control, you might not even be using any ASP.NET controls so in that case if you want to just do a vanilla postback you can do:

document.forms[0].submit();