Promise in Angular Development
The computer language JavaScript was developed to provide interactivity to web pages. The callback
functions in JavaScript coding, however, provide a significant challenge for developers. The reason is
that when visitors interact with a web page, they must cope with asynchronous events.
Many developers found these callback routines to be a hassle, especially when using JavaScript to
create intricate and rich programmes for contemporary browsers. To overcome such complexity, ES6
introduced Promises, a modern and powerful abstraction for writing asynchronous code in a better
and easily maintainable way.
This presentation will explain what a promise is in an Angular application and introduce you to the
idea of JavaScript promises, which were first introduced in ES6.

What is Promise in JavaScript?
A promise is a JavaScript or TypeScript object that may produce a value at some point in time. A
promise may be in one of 4 possible states:
● fulfilled – The action relating to the promise succeeded
● rejected – The action relating to the promise failed just because an error occurs during promise
implementation
● pending – It’s an initial state that hasn’t fulfilled or rejected yet
● settled – Has fulfilled or rejected

How To Create a Promise in JavaScript?
The developers from the leading AngularJS
development company implements Promises by
calling the then() and catch() methods.
● then() method: It takes two callback
functions as parameters and is invoked
when a promise is either resolved or
rejected.
● catch() method: It takes one callback
function and is invoked when an error
occurs.

What is Promise in Angular?
When working with asynchronous actions like an HTTP request, AngularJS developers are sceptical
of when they will receive a response from the server. By default, Javascript is a programming
language that is Synchronous; Blocking; and Single-threaded language.
Simply put, we may state that only one code can be processed by the Javascript engine at a time in a
single thread. Hence, when a thread is blocked, we are unable to execute other blocked code, which
slows down JS and prevents it from performing multiple actions.
On the other hand, code becoming non blocking is entirely the result of the asynchronous activity.
That means that it enables developers to run many pieces of code simultaneously.

In the Angular framework, the developers can quickly achieve asynchronous by utilising the following
method.
● The Callback function
● The Promise from Javascript can be used in Angular.
● Observable using Rxjs library in Angular: Library for reactive programming using Observables.
Additionally, developers can swiftly shift the code to the following line by allowing the user interface to
stay responsive while dealing with an asynchronous activity. As HTTP requests and other
asynchronous actions are carried out in the background, the user keeps working on the application.
Now Angular's Promise notion enters the scene. Javascript's promise is just a task wrapped in an
asynchronous operation that receives notifications whenever it is successful or unsuccessful in the
future.
As the developer executes the Promise, one of two things can happen: the Promise is successful or
fails. Developers may deal with easy-to-read and write code using the async and await keywords with
Promise, and we can also utilise "try and catch" for error management (async & await).

Angular Promise Example
Time to examine promises' use in Angular development. With Angular CLI, we will start by building a
straightforward Angular application.
ng new angular-promise-example –routing=false –style=css
By using this command, a fresh Angular application will be created with no routing and a CSS
stylesheet format. To send API queries to web servers to produce or fetch data, we will next
configure HttpClient.
Since you must always wait a certain length of time before receiving the requested data, fetching
data is a typical asynchronous process. HttpClient can be used in Angular programming to fetch data.
We'll show how to use JavaScript/ES6 Promises in place of RxJS Observables, which are how
HttpClient handles asynchronous operations.

In the app/app.module.ts file of your project, add the following changes:
Next, open the src/app/app.component.ts file and add the following code to send a HTTP GET
request and process the response using a Promise in Angular.

We import HttpClient and inject it via the component constructor and use it to send the HTTP request.
Next, we call the get() method to send the request and the toPromise() method to convert the
returned RxJS Observable to a promise.

In the ngOnInit() life-cycle method, we send the actual request by calling the then() method of the
promise as follows:
If the promise is resolved successfully we simply output the data in the console and in case of an
error we display the error.

Conclusion
I trust that now you have a good understanding of what Promise in Angular development is. In
addition, we saw the ES6-introduced JavaScript Promises concept. JavaScript promises that can be
specifically incorporated into the application have been put into practice.
Also, we came upon the Angular application example of constructing a promise. Nonetheless,
Angular is the greatest and most popular choice if you have any questions about developing web
applications. It enables you to create applications for various business sectors.

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