Introduction to Service worker

Service Workers  are like a JavaScript files , but they have access to set of features and also it’s run on a separate thread. It will not share the thread where the normal JavaScript code loaded by your html pages runs on. When you register a service worker through your  html page or code initially , it will manage all the pages of a given scope. For example if your service worker is attached to the root of your application , then it’s applicable to all the html pages not only the single html page.

Service worker is also live on even after you closed all the pages in the browser .For example if you are using a mobile phone, even if you close the application , service worker will run as a background process.

What we can do with service workers?

Since it’s background process ,service workers can react all kind of incoming events from your JavaScript code as well as from your server as web push notification and can do something based on that , for example you can return a cached asset , show some notifications to the user and so on.

Type of events which service workers can listen to

fetch

By using fetch API you can initiate an http request asynchronously to get some resources across the network. This new API is similar to XmlHttpRequest , but  provides a more powerful and flexible feature set

fetch('https://httpbin.org/get')
.then(function(response) {
return response.json();
})

https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch

 

What you can do with this fetch API in service worker?

By using service worker every http request send via fetch API can intercept like a network proxy.So you can block the request if you want and also you can manipulate the request.

self.addEventListener('fetch', function (event) {

event.respondWith(fetch(event.request));

});

Also  the image tags which point to some image resource in the html will also send a fetch request by the browser. So you can cache the same by intercepting it.

Push Notifications

Every browser like Google Chrome,Mozilla Firefox has its own web push  server , So you can push notifications from your web server to this web push server and then browser will send this notifications to your client. This feature is very useful for a mobile to receive push notifications even if we are not in the application.

https://developers.google.com/web/fundamentals/codelabs/push-notifications/

How to Register a service worker?

Please note one thing  , the scope of the service worker depends on the location of the file. If the file is  on your root of the application , service workers are applies to all pages in your application. If you can create the file in sub folder , service worker could only control the pages inside of this folder.

  • Create a new file , you can give any name you want . Here I gave sw.js
  • Add your service worker registration code , you can add this in to your start up js file.
if ('serviceWorker' in navigator) {
navigator.serviceWorker
.register('/sw.js')
   .then(function () {
       console.log('Service worker registered!');
    })
   .catch(function(err) {
      console.log(err);
    });
}

That’s it .

How Observable works in RxJS?

What is Observable?

Observable is a like a stream which allow to pass zero or more events or values  and you can subscribe each using a callback function.

Why Observables are so important?

In almost all UI when user do some interactions with it you can see the same progression. for example

  • You are listening an event
  • You are making an asynchronous request to the server and may be you are follow up with some transformations or Animations

Observables are capable of modeling all the above things together with a very little code.

In order to understand how Observable works , look at the sample code

var observable = Observable.create(function(observer) {
var sum = 0;
for (var i = 1; i <= 4; i++) {
if (i <= 3) { sum = sum + i; observer.next(i); //You can emit each item from the observable also } if (i === 4) { setTimeout( i => {
observer.next(sum);
observer.complete();
},
5000,
i
);
}
}
});

In this sample code, I am running a for loop and emit each value by using observer.next(value) , when the value of i become 4 you can see emiting the sum of 3 numbers and exiting the all observable sequence by simple calling observable.complete();

Observables are lazy which means , the above code never executes unless you are subscribing it.

Let’s subscribe the Observable to get each value. I am removing the lambda expression to understand more clearly

observable.subscribe({
next: function(x) {
console.log("got value " + x);
},
error: err => console.error("something wrong occurred: " + err),
complete: function() {
//when I become 4 it will complete
console.log("completed");
}
});

In the callback function of next , you will get all the values emitted from the Observable sequence including sum as final value and finally execute the complete callback function.

You can simply receive each value like the below syntax also instead of specify next explicitly

observable.subscribe(function(x) {
//here you will get all the value including sum
console.log(x);
});

Let me tell you one more scenario with the same sample by simply commenting one line of code. I am not emitting each value instead I want to emit the sum only from the observable and complete.

var observable = Observable.create(function(observer) {
var sum = 0;
for (var i = 1; i <= 4; i++) {
if (i <= 3) { sum = sum + i; //commented the code } if (i === 4) { setTimeout( i => {
observer.next(sum);
observer.complete();
},
5000,
i
);
}
}
});

Now when you subscribe , you will have only one value , that is sum

observable.subscribe(function(x) {
//here you will the sum
console.log(x);
});

Official Documenation to learn about RxJS features
https://www.learnrxjs.io/