Reactive programming is a programming paradigm focused on data flows and the propagation of changes. This means that, instead of dealing with values that change over time, as is common in imperative programming, in reactive programming we deal with sequences of events over time. In this chapter, we will discuss reactive programming with RxJS in front-end development.
RxJS is a library for composing asynchronous and event-based programs using observable sequences. It's perfect for handling events, especially when you need to handle a large amount of events arriving over time. RxJS allows you to create a stream of events and react to them declaratively, without worrying about low-level details.
To get started with RxJS, we first need to understand the concept of Observables. An Observable is a representation of any set of values over any amount of time. It can be a single value, a sequence of values, or a continuous stream of values. Observables are the foundation of reactive programming with RxJS.
An Observable by itself does nothing. It needs an Observer to react to the values it emits. An Observer is just an object with three methods: next, error and complete. The next method is called to receive the next value from the Observable, the error method is called if there is an error during value generation and the complete method is called when there are no more values to be generated.
To create an Observable, you can use the RxJS create function. For example, to create an Observable that outputs numbers from 1 to 5, you could do the following:
const source$ = Rx.Observable.create(observer => {
for (let i = 1; i <= 5; i++) {
observer.next(i);
}
observer.complete();
});
To subscribe to an Observable and start receiving values, you use the Observable's subscribe method. For example, to print the values from the Observable we just created, you would do the following:
source$.subscribe(
value => console.log(value),
error => console.error(error),
() => console.log('Complete')
);
In addition to creating Observables from scratch, you can also turn other things into Observables, such as events, promises, arrays, and so on. RxJS provides many utility functions for this, called operators.
Operators are functions that allow you to manipulate Observables in various ways. For example, you can filter values, map values to other values, combine multiple Observables into one, and so on. Operators are the main tool you use to compose logic in RxJS.
To use an operator, you call the Observable's pipe method and pass the operator to it. For example, to filter the even values of our Observable, you could do the following:
const even$ = source$.pipe(
Rx.operators.filter(value => value % 2 === 0)
);
even$.subscribe(value => console.log(value)); // logs 2, 4
In summary, reactive programming with RxJS is a powerful way to handle events and data flows in front-end applications. With Observables and operators, you can express complex logic in a declarative and manageable way.
