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An implementation of Observables for JavaScript. Requires Promises or a Promise polyfill.


npm install zen-observable


import Observable from 'zen-observable';

Observable.of(1, 2, 3).subscribe(x => console.log(x));


new Observable(subscribe)

let observable = new Observable(observer => {
  // Emit a single value after 1 second
  let timer = setTimeout(() => {'hello');
  }, 1000);

  // On unsubscription, cancel the timer
  return () => clearTimeout(timer);

Creates a new Observable object using the specified subscriber function. The subscriber function is called whenever the subscribe method of the observable object is invoked. The subscriber function is passed an observer object which has the following methods:

  • next(value) Sends the next value in the sequence.
  • error(exception) Terminates the sequence with an exception.
  • complete() Terminates the sequence successfully.
  • closed A boolean property whose value is true if the observer's subscription is closed.

The subscriber function can optionally return either a cleanup function or a subscription object. If it returns a cleanup function, that function will be called when the subscription has closed. If it returns a subscription object, then the subscription's unsubscribe method will be invoked when the subscription has closed.


// Logs 1, 2, 3
Observable.of(1, 2, 3).subscribe(x => {

Returns an observable which will emit each supplied argument.


let list = [1, 2, 3];

// Iterate over an object
Observable.from(list).subscribe(x => {
// Convert something 'observable' to an Observable instance
Observable.from(otherObservable).subscribe(x => {

Converts value to an Observable.

  • If value is an implementation of Observable, then it is converted to an instance of Observable as defined by this library.
  • Otherwise, it is converted to an Observable which synchronously iterates over value.


let subscription = observable.subscribe({
  next(x) { console.log(x) },
  error(err) { console.log(`Finished with error: ${ err }`) },
  complete() { console.log('Finished') }

Subscribes to the observable. Observer objects may have any of the following methods:

  • next(value) Receives the next value of the sequence.
  • error(exception) Receives the terminating error of the sequence.
  • complete() Called when the stream has completed successfully.

Returns a subscription object that can be used to cancel the stream.

observable.subscribe(nextCallback[, errorCallback, completeCallback])

let subscription = observable.subscribe(
  x => console.log(x),
  err => console.log(`Finished with error: ${ err }`),
  () => console.log('Finished')

Subscribes to the observable with callback functions. Returns a subscription object that can be used to cancel the stream.


observable.forEach(x => {
  console.log(`Received value: ${ x }`);
}).then(() => {
  console.log('Finished successfully')
}).catch(err => {
  console.log(`Finished with error: ${ err }`);

Subscribes to the observable and returns a Promise for the completion value of the stream. The callback argument is called once for each value in the stream.


Observable.of(1, 2, 3).filter(value => {
  return value > 2;
}).subscribe(value => {
// 3

Returns a new Observable that emits all values which pass the test implemented by the callback argument.

Returns a new Observable that emits the results of calling the callback argument for every value in the stream.

Observable.of(1, 2, 3).map(value => {
  return value * 2;
}).subscribe(value => {
// 2
// 4
// 6

observable.reduce(callback [,initialValue])

Observable.of(0, 1, 2, 3, 4).reduce((previousValue, currentValue) => {
  return previousValue + currentValue;
}).subscribe(result => {
// 10

Returns a new Observable that applies a function against an accumulator and each value of the stream to reduce it to a single value.


Observable.of(1, 2, 3).concat(
  Observable.of(4, 5, 6),
  Observable.of(7, 8, 9)
).subscribe(result => {
// 1, 2, 3, 4, 5, 6, 7, 8, 9

Merges the current observable with additional observables.

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