# BroadcastChannel API [BroadcastChannel API](https://developer.mozilla.org/en-US/docs/Web/API/Broadcast_Channel_API) is a new API used to communicate between same-origin tabs opened by the same user. Why === Let's say you open two GitHub tabs, the [rust repository](https://github.com/rust-lang/rust) and [your stars](https://github.com/stars) page. You decide to star the awesome rust repository, but then you have to refresh your stars page to see your new star. That's sad. There must be a way for GitHub to refresh your stars page in case you star something in another tab, right? ![Broadcast Channels, SATISFIED](/img/broadcast-channels.jpg) Show me something! ================== Okay, open another page of my blog in a new tab. Now open up your console and enter this: {% highlight javascript %} channel.postMessage('Anybody hears me?'); {% endhighlight %} Boom. I've put this code in my ``, pretty simple: {% highlight javascript %} var channel = new BroadcastChannel('egg'); channel.addEventListener('message', message => { alert('Got a message from the other tab:\n' + message.data); // Egg document.querySelector('header').classList.add('channel'); }); {% endhighlight %} How === BroadcastChannels are pretty easy, here I'm going over the small details. Creating channels ----------------- BroadcastChannels are constructed with a single argument, their name. Browsing contexts should use this name to communicate over a specified channel. There's no limit to how many channels you can create. In the first sentence of article I said it's used to communicate between tabs, but it's actually "browsing contexts". [Browsing contexts](http://www.w3.org/TR/html5/browsers.html#browsing-context) are any environments owning a `Document`, e.g. tabs, windows, iframes, etc. {% highlight javascript %} var channel = new BroadcastChannel('star'); {% endhighlight %} Channels have only one property, `name`. Methods ------- Channels have two methods: ### #postMessage(data: Anything) This is the method used to broadcast a message to everyone subscribed to this channel. `data` can be any kind of Object. ### #close() This method is used to leave a channel, in case you don't want to hear from the channel anymore. Try leaving `channel` from my blog pages, and posting messages with others. Events ------ Channels inherit from [`EventTarget`](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget), so you can use `addEventListener`, `removeEventListener` and `dispatchEvent` methods. Channels have only one event: ### message The event object passed to this event is a [`MessageEvent`](https://developer.mozilla.org/en-US/docs/Web/API/MessageEvent) with the `data` property set to the actual message sent using `postMessage`. --- Another example =============== Let's Fix GitHub ---------------- Okay, let's try something cool, I promise you will love it. Open a browser with [BroadcastChannel support](http://caniuse.com/#feat=broadcastchannel) and Install [GreaseMonkey](http://www.greasespot.net/). You have to add two scripts, one for repository pages, and one for the stars page. Click "New User Script", and fill the inputs like this: ![GreaseMonkey New User Script](/img/greasemonkey-new-user-script.png) ![GreaseMonkey New Script](/img/greasemonkey-github-stars.png) In the Script Editor, enter this: {% highlight javascript %} // join the "Stars" channel var channel = new BroadcastChannel('Stars'); // reload the page when someone from a github.com tab broadcasts a message channel.addEventListener('message', message => { location.reload(); }); {% endhighlight %} Now create another script and fill the form like this: ![GreaseMonkey New Script](/img/greasemonkey-github-repositories.png) Use this code: {% highlight javascript %} // join the "Stars" channel var channel = new BroadcastChannel('Stars'); // once the star/unstar forms are submitted, broadcast a message to // all github.com tabs $('.starring-container form').submit(e => { channel.postMessage('refresh'); }); {% endhighlight %} Save, refresh your pages and Voila! You can now Star/Unstar any repository while your stars page is open, and your stars page will refresh immediately. --- That's it, I really like this API as it's best of both worlds, simple and useful. There are a lot of things which can be done with help of BroadcastChannels, I would love to hear your feedback on this API.