Rxjs Transformation Operators

RxJS is a library for composing asynchronous and event-based programs by using observable sequences. It provides one core type, the Observable, satellite types (Observer, Schedulers, Subjects) and operators inspired by Array methods (map, filter, reduce, every, etc) to allow handling asynchronous events as collections.

ReactiveX combines the Observer pattern with the Iterator pattern and functional programming with collections to fill the need for an ideal way of managing sequences of events.

The essential concepts in RxJS which solve async event management are:

• Observable: represents the idea of an invokable collection of future values or events.

• Observer: is a collection of callbacks that knows how to listen to values delivered by the Observable.

• Subscription: represents the execution of an Observable, is primarily useful for cancelling the execution.

• Operators: are pure functions that enable a functional programming style of dealing with collections with operations like map, filter, concat, reduce, etc.

• Subject: is equivalent to an EventEmitter, and the only way of multicasting a value or event to multiple Observers.

• Schedulers: are centralized dispatchers to control concurrency, allowing us to coordinate when computation happens on e.g. setTimeout or requestAnimationFrame or others.

Operators are functions. There are two kinds of operators:

Pipeable Operators are the kind that can be piped to Observables using the syntax observableInstance.pipe(operator()). These include, filter(...), and mergeMap(...). When called, they do not change the existing Observable instance. Instead, they return a new Observable, whose subscription logic is based on the first Observable.

A Pipeable Operator is a function that takes an Observable as its input and returns another Observable. It is a pure operation: the previous Observable stays unmodified.

A Pipeable Operator is essentially a pure function which takes one Observable as input and generates another Observable as output. Subscribing to the output Observable will also subscribe to the input Observable.

Creation Operators are the other kind of operator, which can be called as standalone functions to create a new Observable. For example: of(1, 2, 3) creates an observable that will emit 1, 2, and 3, one right after another. Creation operators will be discussed in more detail in a later section.