Observable

A representation of any set of values over any amount of time. This is the most basic building block of RxJS.

class Observable<T> implements Subscribable { static create: Function static if: typeof iif static throw: typeof throwError constructor(subscribe?: (this: Observable<T>, subscriber: Subscriber<T>) => TeardownLogic) _isScalar: boolean source: Observable<any> operator: Operator<any, T> lift<R>(operator: Operator<T, R>): Observable<R> subscribe(observerOrNext?: PartialObserver<T> | ((value: T) => void), error?: (error: any) => void, complete?: () => void): Subscription _trySubscribe(sink: Subscriber<T>): TeardownLogic forEach(next: (value: T) => void, promiseCtor?: PromiseConstructorLike): Promise<void> pipe(...operations: OperatorFunction<any, any>[]): Observable<any> toPromise(promiseCtor?: PromiseConstructorLike): Promise<T> }

Subclasses

  • ConnectableObservable
  • GroupedObservable
  • Subject
    • BehaviorSubject
    • ReplaySubject
    • AsyncSubject

Static Properties

PropertyTypeDescription
create

Creates a new cold Observable by calling the Observable constructor

if
throw

Constructor

constructor(subscribe?: (this: Observable<T>, subscriber: Subscriber<T>) => TeardownLogic)

Parameters

subscribe

Optional. Default is undefined.

the function that is called when the Observable is initially subscribed to. This function is given a Subscriber, to which new values can be nexted, or an error method can be called to raise an error, or complete can be called to notify of a successful completion.

Properties

PropertyTypeDescription
_isScalar

Internal implementation detail, do not use directly.

source
operator

Methods

lift()

Creates a new Observable, with this Observable as the source, and the passed operator defined as the new observable's operator.

lift<R>(operator: Operator<T, R>): Observable<R>

Parameters

operator

the operator defining the operation to take on the observable

Returns

Observable<R>: a new observable with the Operator applied

subscribe()

Invokes an execution of an Observable and registers Observer handlers for notifications it will emit.

5 overloads...

subscribe(observer?: PartialObserver<T>): Subscription

Parameters

observer

Optional. Default is undefined.

Type: PartialObserver.

Returns

Subscription


subscribe(next: null | undefined, error: null | undefined, complete: () => void): Subscription

Parameters

next

Type: null | undefined.

error

Type: null | undefined.

complete

Type: () => void.

Returns

Subscription


subscribe(next: null | undefined, error: (error: any) => void, complete?: () => void): Subscription

Parameters

next

Type: null | undefined.

error

Type: (error: any) => void.

complete

Optional. Default is undefined.

Type: () => void.

Returns

Subscription


subscribe(next: (value: T) => void, error: null | undefined, complete: () => void): Subscription

Parameters

next

Type: (value: T) => void.

error

Type: null | undefined.

complete

Type: () => void.

Returns

Subscription


subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): Subscription

Parameters

next

Optional. Default is undefined.

Type: (value: T) => void.

error

Optional. Default is undefined.

Type: (error: any) => void.

complete

Optional. Default is undefined.

Type: () => void.

Returns

Subscription

Use it when you have all these Observables, but still nothing is happening.

subscribe is not a regular operator, but a method that calls Observable's internal subscribe function. It might be for example a function that you passed to Observable's constructor, but most of the time it is a library implementation, which defines what will be emitted by an Observable, and when it be will emitted. This means that calling subscribe is actually the moment when Observable starts its work, not when it is created, as it is often the thought.

Apart from starting the execution of an Observable, this method allows you to listen for values that an Observable emits, as well as for when it completes or errors. You can achieve this in two of the following ways.

The first way is creating an object that implements Observer interface. It should have methods defined by that interface, but note that it should be just a regular JavaScript object, which you can create yourself in any way you want (ES6 class, classic function constructor, object literal etc.). In particular do not attempt to use any RxJS implementation details to create Observers - you don't need them. Remember also that your object does not have to implement all methods. If you find yourself creating a method that doesn't do anything, you can simply omit it. Note however, if the error method is not provided, all errors will be left uncaught.

The second way is to give up on Observer object altogether and simply provide callback functions in place of its methods. This means you can provide three functions as arguments to subscribe, where the first function is equivalent of a next method, the second of an error method and the third of a complete method. Just as in case of Observer, if you do not need to listen for something, you can omit a function, preferably by passing undefined or null, since subscribe recognizes these functions by where they were placed in function call. When it comes to error function, just as before, if not provided, errors emitted by an Observable will be thrown.

Whichever style of calling subscribe you use, in both cases it returns a Subscription object. This object allows you to call unsubscribe on it, which in turn will stop the work that an Observable does and will clean up all resources that an Observable used. Note that cancelling a subscription will not call complete callback provided to subscribe function, which is reserved for a regular completion signal that comes from an Observable.

Remember that callbacks provided to subscribe are not guaranteed to be called asynchronously. It is an Observable itself that decides when these functions will be called. For example of by default emits all its values synchronously. Always check documentation for how given Observable will behave when subscribed and if its default behavior can be modified with a scheduler.

Example

Subscribe with an Observer

const sumObserver = { sum: 0, next(value) { console.log('Adding: ' + value); this.sum = this.sum + value; }, error() { // We actually could just remove this method, }, // since we do not really care about errors right now. complete() { console.log('Sum equals: ' + this.sum); } }; Rx.Observable.of(1, 2, 3) // Synchronously emits 1, 2, 3 and then completes. .subscribe(sumObserver); // Logs: // "Adding: 1" // "Adding: 2" // "Adding: 3" // "Sum equals: 6"

Subscribe with functions

let sum = 0; Rx.Observable.of(1, 2, 3) .subscribe( function(value) { console.log('Adding: ' + value); sum = sum + value; }, undefined, function() { console.log('Sum equals: ' + sum); } ); // Logs: // "Adding: 1" // "Adding: 2" // "Adding: 3" // "Sum equals: 6"

Cancel a subscription

const subscription = Rx.Observable.interval(1000).subscribe( num => console.log(num), undefined, () => console.log('completed!') // Will not be called, even ); // when cancelling subscription setTimeout(() => { subscription.unsubscribe(); console.log('unsubscribed!'); }, 2500); // Logs: // 0 after 1s // 1 after 2s // "unsubscribed!" after 2.5s

_trySubscribe()

_trySubscribe(sink: Subscriber<T>): TeardownLogic

Parameters

sink

Type: Subscriber.

Returns

TeardownLogic

forEach()

forEach(next: (value: T) => void, promiseCtor?: PromiseConstructorLike): Promise<void>

Parameters

next

a handler for each value emitted by the observable

promiseCtor

Optional. Default is undefined.

a constructor function used to instantiate the Promise

Returns

Promise<void>: a promise that either resolves on observable completion or rejects with the handled error

pipe()

Used to stitch together functional operators into a chain.

11 overloads...

pipe(): Observable<T>

Parameters

There are no parameters.

Returns

Observable<T>


pipe<A>(op1: OperatorFunction<T, A>): Observable<A>

Parameters

op1

Type: OperatorFunction.

Returns

Observable<A>


pipe<A, B>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>): Observable<B>

Parameters

op1

Type: OperatorFunction.

op2

Type: OperatorFunction.

Returns

Observable<B>


pipe<A, B, C>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>): Observable<C>

Parameters

op1

Type: OperatorFunction.

op2

Type: OperatorFunction.

op3

Type: OperatorFunction.

Returns

Observable<C>


pipe<A, B, C, D>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>, op4: OperatorFunction<C, D>): Observable<D>

Parameters

op1

Type: OperatorFunction.

op2

Type: OperatorFunction.

op3

Type: OperatorFunction.

op4

Type: OperatorFunction.

Returns

Observable<D>


pipe<A, B, C, D, E>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>, op4: OperatorFunction<C, D>, op5: OperatorFunction<D, E>): Observable<E>

Parameters

op1

Type: OperatorFunction.

op2

Type: OperatorFunction.

op3

Type: OperatorFunction.

op4

Type: OperatorFunction.

op5

Type: OperatorFunction.

Returns

Observable<E>


pipe<A, B, C, D, E, F>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>, op4: OperatorFunction<C, D>, op5: OperatorFunction<D, E>, op6: OperatorFunction<E, F>): Observable<F>

Parameters

op1

Type: OperatorFunction.

op2

Type: OperatorFunction.

op3

Type: OperatorFunction.

op4

Type: OperatorFunction.

op5

Type: OperatorFunction.

op6

Type: OperatorFunction.

Returns

Observable<F>


pipe<A, B, C, D, E, F, G>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>, op4: OperatorFunction<C, D>, op5: OperatorFunction<D, E>, op6: OperatorFunction<E, F>, op7: OperatorFunction<F, G>): Observable<G>

Parameters

op1

Type: OperatorFunction.

op2

Type: OperatorFunction.

op3

Type: OperatorFunction.

op4

Type: OperatorFunction.

op5

Type: OperatorFunction.

op6

Type: OperatorFunction.

op7

Type: OperatorFunction.

Returns

Observable<G>


pipe<A, B, C, D, E, F, G, H>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>, op4: OperatorFunction<C, D>, op5: OperatorFunction<D, E>, op6: OperatorFunction<E, F>, op7: OperatorFunction<F, G>, op8: OperatorFunction<G, H>): Observable<H>

Parameters

op1

Type: OperatorFunction.

op2

Type: OperatorFunction.

op3

Type: OperatorFunction.

op4

Type: OperatorFunction.

op5

Type: OperatorFunction.

op6

Type: OperatorFunction.

op7

Type: OperatorFunction.

op8

Type: OperatorFunction.

Returns

Observable<H>


pipe<A, B, C, D, E, F, G, H, I>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>, op4: OperatorFunction<C, D>, op5: OperatorFunction<D, E>, op6: OperatorFunction<E, F>, op7: OperatorFunction<F, G>, op8: OperatorFunction<G, H>, op9: OperatorFunction<H, I>): Observable<I>

Parameters

op1

Type: OperatorFunction.

op2

Type: OperatorFunction.

op3

Type: OperatorFunction.

op4

Type: OperatorFunction.

op5

Type: OperatorFunction.

op6

Type: OperatorFunction.

op7

Type: OperatorFunction.

op8

Type: OperatorFunction.

op9

Type: OperatorFunction.

Returns

Observable<I>


pipe<A, B, C, D, E, F, G, H, I>(op1: OperatorFunction<T, A>, op2: OperatorFunction<A, B>, op3: OperatorFunction<B, C>, op4: OperatorFunction<C, D>, op5: OperatorFunction<D, E>, op6: OperatorFunction<E, F>, op7: OperatorFunction<F, G>, op8: OperatorFunction<G, H>, op9: OperatorFunction<H, I>, ...operations: OperatorFunction<any, any>[]): Observable<{ }>

Parameters

op1

Type: OperatorFunction.

op2

Type: OperatorFunction.

op3

Type: OperatorFunction.

op4

Type: OperatorFunction.

op5

Type: OperatorFunction.

op6

Type: OperatorFunction.

op7

Type: OperatorFunction.

op8

Type: OperatorFunction.

op9

Type: OperatorFunction.

operations

Type: OperatorFunction[].

Returns

Observable<{ }>

toPromise()

3 overloads...

toPromise<T>(): Promise<T>

Parameters

There are no parameters.

Returns

Promise<T>


toPromise<T>(PromiseCtor: typeof Promise): Promise<T>

Parameters

PromiseCtor

Type: typeof Promise.

Returns

Promise<T>


toPromise<T>(PromiseCtor: PromiseConstructorLike): Promise<T>

Parameters

PromiseCtor

Type: PromiseConstructorLike.

Returns

Promise<T>