Gain insights into reactive programming with RxJS 7, explore data streams, error handling, and multicasting to build a recipe app, and learn to test and optimize reactive applications.

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The reactive programming paradigm is designed to make working with asynchronous events and data streams convenient. 

This course teaches you about the reactive programming paradigm and when to choose it over others. You’ll learn to use new RxJS 7 features to create a recipe book web application that lets users browse and share recipes. In this context, you’ll learn about data streams, error handling, and how to combine streams and transform one data stream into another. You’ll develop an understanding of multicasting and caching streams, how to share data between components, perform bulk operations such as file uploads, and perform real-time updates. Finally, you’ll learn how to test a reactive application. To assess your knowledge, there’s a project, coding challenges, and assessments.

After this course, you’ll be able to determine the utility of the reactive paradigm for any application and have mastery over new RxJS 7 features to create robust reactive applications.

Reactive Programming with Angular and RxJS 7

# 
One of the major deprecations of RxJS 7 is `toPromise()`, which will be deleted permanently after the release of RxJS 8. So it's highly recommended that we avoid using `toPromise()` in future development.

Now let's give a little bit of background to understand the reason behind this deprecation. It's important to understand the use case of `toPromise()`, why this particular method is available, and when to use it. Additionally, it's important to understand the incoherent behavior of this method in RxJS 6 in order to anticipate problems that might arise.

# What is a promise?

Promises are used by many developers nowadays, and there are thousands of promise-based libraries. A **promise** is an object that holds the result (or failure) of an asynchronous operation.

Promises have been available in JavaScript through third-party libraries such as jQuery. Additionally, ECMAScript 6 added built-in support to JavaScript for promises.

The common key point between observables and promises is that both objects could produce values over time. However, the major difference is that observables can produce no value or more than one, while promises only produce one value when resolved successfully.

The `toPromise()` method available in RxJS is a utility method that is used to convert an observable to a promise. The `toPromise()` subscribes to the observable and returns the last value emitted by this observable.

In RxJS 6, when an error happens, `toPromise()` returns `undefined`. But this is not accurate!

That's because we cannot differentiate between the case where no value was emitted before the completion of the observable and the case where an undefined value was emitted last.

# Examples

Let's look at the first example in the following snippet. Here, `toPromise()` will return the last value emitted by the observable, which is "World":


Learn about the deprecation of the toPromise() method and the functions that replace it.

Understanding the toPromise() Deprecation

Getting Started

The Power of the Reactive Paradigm

RxJS 7—The Major Features

A Walk-Through of the Application

Fetching Data as Streams

Error Handling

Combining Streams

Assessment 1

Transforming Streams

Multicasting Essentials

Caching Streams

Sharing Data between Components

Bulk Operations

Processing Real-Time Updates

Testing RxJS Observables

Assessment 2

Conclusion

Understanding the toPromise() Deprecation

What is a promise?