crosstab

A utility library for cross-tab communication using localStorage.

How can I get it?

crosstab is available through bower and can be installed with the following command:

bower install crosstab

What is it?

crosstab is a javascript utility library for inter-tab communication on the same domain. It offers the following features:

• Tracks open tabs
• Designates a master tab and updates the master tab if it closes or times out. This is useful for maintaining a single server connection across all tabs.
• Broadcast messages to all tabs or a particular tab

Browser Compatibility

Browser	Version Tested
IE	9+
Chrome	35+
FireFox	30+
Safari	6.1+

API

Most of the time, you will only need to do two things with crosstab, set up event handlers, and fire off events.

Broadcasts

crosstab broadcasts messages like this:

crosstab.broadcast(event, data, destination);

If a destination is not specified, the message is broadcast to all tabs, including itself.

Event Handlers

crosstab registers event handlers like this:

crosstab.on('eventName', function(message) {
    // Handle event
});

The messages received by events have the following format:

var message = {
    event: event,             // The name of the event
    data: data,               // The data to pass
    destination: destination, // The destination tab
    origin: crosstab.id,      // The origin tab
    timestamp: util.now()     // The time the message was created
};

The event will not fire if the destination is present and differs from the id of the current tab.

Why was it made?

I wanted to be able to have robust cross tab communication for the purpose of resource sharing (such as websockets). Though there are some libraries which have a similar goal, they all had subtle issues. This library aims to be the most correct it can be for supported browsers. This library was created with inspiration from the excellent intercom.js library, and addresses several of it's shortcomings:

• intercom.js doesn't implement proper locking.
• does not guarantee that one tab holds the lock at a time (in fact this is impossible to guarantee flat out, but can be guaranteed within defined execution times).
• locking on functions that throw will break.
• Updates to any localStorage item will cause the locks to attempt to be acquired instead of only removals of the lock.
• in trying to support IE8 message broadcasting in intercom.js has a race condition where messages can be dropped.
• intercom.js leaks memory by maintaining a state of every message id received (also in an attempt to support IE8)

crosstab solves these issues by dropping support for IE8 and using a lockless system that is entirely event driven (IE8 cannot pass messages via localStorage events, which is why intercom.js requires locking, because it supports IE8).