How to change browser focus from one tab to another

I have a JavaScript chat client running in one browser tab (call it tab A). Now when a message arrives the user might be on another browser tab (call it tab B). I'm looking for ways to change the focus from tab B to my chat client (tab A) when such a message arrives.

I could not find a way to do this.

It is not possible - due to security concerns.

unless by "tab" you mean a window and a popup window that (due to browser preferences) opened up in a new tab. If this is the case, then yes you can.

//focus opener... from popup
window.opener.focus();

//focus popup... from opener
yourPopupName.focus(); 

It is possible to shift focus back to Tab A by means of an alert in Tab A e.g. alert('New Message')

However, you need to be careful using this as it is very likely to annoy people. You should only use it if you make it optional in your app. Otherwise, updating Tab A's title and/or the favicon would appear to be best as nc3b says.

The best you could would probably be to change the title of the page alerting the user the tab needs attention (maybe also the favicon - look at how meebo does it, it's really annoying but effective)

Chrome and firefox now have notifications. I think notifications are probably a more user friendly way to alert the user that something has changed on your app than popping an alert and forcing them to your page.