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Can I Send a Web Notification on a Specific Time?

I am trying to implement notifications that do not need the interaction of the backend to be shown. The use of this is to put a remind me button, that would send a notification on certain time of the day (that the user specified). I am pretty new to service workers but I understand that they work asynchronously. That being said, is there any way in which I could do something like the following pseudocode?
function timeLeft(time){
setTimeout(() => showNotification(), time);
}
This would work if I put it on a regular javascript file and the user has the browser still open.
It does not need to be exactly like that, it just needs to solve my problem. Thank you un advance.

First you need to have an ID for each browser. If you use the Push API, then you can use the browser endpoint.
When a user adds a reminder, you must store the tuple (endpoint, send_at, reminder) in your database on your server
Run a cron job on your server (or create a scheduled job with Sidekiq, etc.) in order to send the reminder at the correct time (using the Push API)
Otherwise you can use a service which allows you to create scheduled notifications: see this example on my blog.

How to notify user that new channel needs to be opened using Pusher?

I use pusher to implement real-time chat and notifications in my website. Users are allowed to start conversation(chat) with others and conversation can be only 1 on 1.
Messages are also stored in db so that user could have chat history as well.
I user with userId=14 has three active conversation I would generate JS script in php that would connect him to pusher and subscribe for three presence-channels representing each of three conversations. Every channel has events like "new-msg" etc. Everything work great - users get message if chat window is open or notification badge if not.
The problem starts when another user want to start a new conversation with userId=14. There is no private or presence channel for that. If I used public channel then every logged in user would get notified which is not good.
What I do know is that once logged in my website users connect to pusher and additionally to already active presence-channels subsribe to public channel "new-conversations" but every user waits for specific event which represents his userId.
var pusher = new Pusher('APP_KEY');
var channel = pusher.subscribe('new-conversations');
channel.bind(userId,
function(data) {
// subscribes to new presence-channel using information from data variable
}
);
When another user wants to start conversation with userId=14 he makes ajax call to server where PHP script generates event representing userId=14 containing data about himself and sends it to every connected(logged in) user but only userId=14 is waiting for this. Once triggered userId=14 can subsribe to new presence-channel.
While all this scenario works, I was wondering if there is some better, more cleaner out-of-the-box solution to this situation? I have read through pusher documentation but found nothing but I refuse to except that pusher developrs havent though about it.
POSSIBLE SOLUTION
I though of another solution. Every user should subscribe to presence-channel var channel = pusher.subscribe('presence-14'); where 14 = userId. If userId= 39 wants to start conversation with him he would send userId=14 to server and server would send userId=39 to channel "presence-14". Is that it?

WebRTC: Unable to successfully complete signalling process using DataChannel

I've been having trouble establishing a WebRTC session and am trying to simplify the issue as much as possible. So I've written up a simple copy & paste example, where you just paste the offer/answer into webforms and click submit.
The HTML+JS, all in one file, can be found here: http://pastebin.com/Ktmb3mVf
I'm on a local network, and am therefore removing the ICE server initialisation process to make this example as bare-bones as possible.
Here are the steps I'm carrying out in the example:
Page 1
Page 1 (loads page), enters a channel name (e.g. test) and clicks create.
A new Host object is created, new PeerConnection() and createDataChannel are called.
createOffer is called, and the resulting offerSDP is pasted into the offer textarea.
Page 2
Copy offerSDP from Page 1 and paste into offer textarea on Page 2, click join.
New Guest object is created, PeerConnection and an ondatachannel handler is set.
setRemoteDescription is called for the Guest object, with the offerSDP data.
createAnswer is called and the result is pasted into the answer textarea box.
Page 1
The answerSDP is copied from Page 2 and pasted into the answer textarea of Page 1, submit answer is clicked.
Host.setRemoteDescription is called with the answerSDP data. This creates a SessionDescription, then peer.setRemoteDescription is called with the resulting data.
Those are the steps carried out in the example, but it seems I'm missing something critical. After the offerer's remoteDescription is set with the answerSDP, I try to send a test message on the dataChannel:
Chrome 40
"-- complete"
> host.dataChannel.send('hello world');
VM1387:2 Uncaught DOMException: Failed to execute 'send' on 'RTCDataChannel': RTCDataChannel.readyState is not 'open'
Firefox 35
"-- complete"
ICE failed, see about:webrtc for more details
> host.dataChannel.send('hello world');
InvalidStateError: An attempt was made to use an object that is not, or is no longer, usable
I also had a more complicated demo operating, with a WebSocket signalling server, and ICE candidates listed, but was getting the same error. So I hope this simplification can help to track down the issue.
Again, the single-file code link: http://pastebin.com/Ktmb3mVf

To enable webRTC clients to connect to each other, you need ICE. While STUN and TURN, which you don't need for such a test, are part of that, even without these helpers you still need to use ICE to tell the other end which IP/port/protocol to connect to.
There are two ways to do this: Google's "trickle ice", where the SDP (answer/offer) is passed on without any ICE candidates. These are then transported over a separate signaling layer and added as they are discovered. This speeds up the connection process, as ICE takes time and some late ICE candidates might not be needed.
The classic method is to wait until all ICE candidates have been gathered, and then generate the SDP with these already included.
I have modified your latest version to do that: http://pastebin.com/g2YVvrRd
You also need to wait for the datachannel/connection to become available before being able to use it, so I've moved the sending of the message to the channels onopen event.
The significant changes to the original code:
The interface callbacks were removed from Host.prototype.createOffer and Guest.prototype.createAnswer, instead we attach the provided callback function to the respective objects for later use.
self.cb = cb;
Both Host and Guest have an added ICE handler for the PeerConnection:
var self = this;
this.peer.onicecandidate = function (event) {
// This event is called for every discovered ICE candidate.
// If this was trickle ICE, you'd pass them on here.
// An event without an actual candidate signals the end of the
// ICE collection process, which is what we need for classic ICE.
if (!event.candidate) {
// We fetch the up to date description from the PeerConnection
// It now contains lines with the available ICE candidates
self.offer = self.peer.localDescription;
// Now we move on to the deferred callback function
self.cb(self.offer);
}
}
For the guest self.offer becomes self.answer
The interface handler $("#submitAnswer").click() does not send the message anymore, instead it is send when the datachannel is ready in the onopen event defined in setChannelEvents().
channel.onopen = function () {
console.log('** channel.onopen');
channel.send('hello world!');
};

RTCSessionDescription and video/audio broadcast

Once I have exchanged session description between two peers. How can I allow the user to prevent audio and/or video broadcast? Do I need to exchange the Session Descriptions again?

"Broadcast" is probably not the correct term since PeerConnections are always unicast peer-to-peer.
To acquire an audio/video stream from the user's devices you call getUserMedia() and to send these to the other peer you call addStream() on the PeerConnection object.
So to allow the user to not send the acquired stream just let her choose whether to call addStream() or not. E.g. show a popup saying "Send Audio/Video to the other user?". If she chooses "Yes" call addStream() on the PeerConnection object, otherwise just don't call it.
EDIT to answer question in comment:
If you'd like to stop sending of audio and/or video just call removeStream() on the PeerConnection object with the stream to remove as parameter. This will per the API spec trigger a renegotiation.
See http://dev.w3.org/2011/webrtc/editor/webrtc.html#interface-definition for further details.

Sending data only to chosen users using Socket.io-node

Is it possible to send data using socket.io-node just to chosen group of users? For example, how could I implement chat with different rooms? I dont want .broadcast() to send data to all logged in users.

Normally you should have for each room a list of connected user and those user all have a client object that you should have stored somewhere. So when you want to send a message to a specific room, you just have to iterate over the connected user of that room and access their client object and send the data.
In short, it is possible you just have to send your data to each of the users in the group one-by-one.

socket.io has a grouping functionality built in
On the socket object for a single connection, like you get passed when a new user connects, you can call .join('roomName') where roomName is any string you want to use to identify the "room", you could use a room name like "profile/14" to create a channel for updates to user #14's profile.
Then on the main io object do something like:
io.sockets.in('profile/14').emit('newComment', {message:'hello'});
The message will go out to all connections that have .join()'d the given room.
Typically I'll have my client emit a "hello" event onConnect that identifies what content the client is interested in subscribing to, and then on the server side my handler for the "hello" event handles .join()'ing the client into whatever rooms are needed