Observable
An observable is a source that emits a sequence of values or notifications over time to consumers that subscribe to it.
The Joy of JavaScript
An Observable is a push-based abstraction that represents a stream of values or events over time and can be subscribed to for delivery of those emissions.
- How it is used
- Provides a consistent publish/subscribe interface for processing values from varied push-based sources.
- How deep it goes
- Works through subscription, cleanup, custom sources, and operator implementation; leaves a full operator toolkit to libraries.
- Examples
- Uses subscribed words, timer-generated random numbers, mapped number sequences, and a file-block validation pipeline.
- What you can do after
- Build it from scratch by wrapping sources and composing custom operators into subscribed processing pipelines.
Functional Programming in JavaScript
An object representing a stream of values or events that observers can subscribe to over time.
- How it is used
- Unifies asynchronous and event-based data so functional operations can process user input and other sources consistently.
- How deep it goes
- Defines streams, subscription, transformation, errors, and completion, while deferring broader reactive operations to later treatment.
- Examples
- Demonstrates numeric ranges, generated squares, odd-number transformation, SSN events, and student data from a promise.
- What you can do after
- Apply RxJS to build composable pipelines for asynchronous values and event streams.
Read how Functional Programming in JavaScript explains observable →
Modern Angular
An observable is a computation or data source that emits a sequence of values or notifications over time to subscribed consumers.
- How it is used
- Handles asynchronous HTTP results, framework events, reactive-form changes, and shared changing state.
- How deep it goes
- Works through creation, combination, and map transformations, then explains signal conversion timing and injection-context constraints.
- Examples
- Uses authentication status, numeric of/merge/map streams, a ticking count signal, and time-off request actions.
- What you can do after
- Apply it via RxJS to expose read-only state, transform event streams, and bridge signals with toObservable.
Pro Angular 16
An observable is a reactive abstraction representing a sequence of values that can be emitted asynchronously over time.
- How it is used
- Represents asynchronous HTTP results and ongoing change notifications in Angular applications.
- How deep it goes
- Defines Observable<T> and subscribe callbacks through a ticker, then applies subscriptions to framework APIs without explaining RxJS internals.
- Examples
- Uses an interval ticker, REST-backed order saving, form input changes, and content-query notifications.
- What you can do after
- Apply it via RxJS and Angular APIs to subscribe to emitted values and convert an observable to a signal when needed.
Get Programming with JavaScript Next: New features of ECMAScript 2015, 2016, and beyond
An observable is a source of asynchronous values that can be subscribed to and can signal start, next, error, and complete notifications to an observer.
- How it is used
- Supplies subscribable streams for recurring values and push-based updates, illustrated by a clock widget.
- How deep it goes
- Walks through construction, notifications, cleanup, and composition, then sketches a filter combinator without forwarding error or completion.
- Examples
- Uses a ticking clock, an iterable converted to a stream, odd-number filtering, and a cumulative-sum exercise.
- What you can do after
- Build it from scratch with the Observable constructor, subscribe and unsubscribe, and compose or write combinators.
Related concepts
- toSignalused by · toSignal is an Angular interoperability function that creates a signal reflecting the latest value emitted by an observable.
- promisealternative to · A JavaScript Promise is an object representing the eventual completion or failure of an asynchronous operation and its resulting value or reason.
- HTTP clientproduced by · An HTTP client is software or an API component that sends HTTP requests and receives and processes HTTP responses.
- RxJSused by · RxJS is a JavaScript library implementing ReactiveX with observables and operators for composing asynchronous and event-based programs.
- signalcontrasts with · A signal is an observable value, event, or pattern that conveys information about a condition, state, or phenomenon.
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