Whenever a tab modifies something in localStorage, an event fires in every other tab. This means we're able to communicate across browser tabs simply by setting values on localStorage. post
The upcoming SharedWorker API allows to transmit data across iframes and even browser tabs or windows. It landed in Chrome years ago, and not so long ago in Firefox, but it's nowhere to be seen in IE or Safari. A wildly supported alternative exists that can be used today, but it's largely unknown.