ASP.NET Javascript and postback

I have a page located in an iframe of another page. It calls a js function like this:

function F(a)  
{  
    parent.document.getElementById('A').value = a;
    parent.document.getE开发者_如何学ClementById('B').click();
}

It results in page reloading. How can I prevent it? Previously I've used return false; and it worked. But not in this case.

F("some value"); return false; after calling the function not inside.