February 22, 2018

Testing Observables With Marble

Marble diagrams

Marble diagrams are a way to visually represent Observables. The marbles represent a value being emitted, the passage of time is represented from left to right, a vertical line represents the completion of an Observable, and an X represents an error.

  • - represents the passage of 10 frames of time.
  • | represents the completion of an Observable.
  • ^ represents the subscription point of an Observable (only valid for hot Observables).
  • # represents an error. The value of the error can be provided to errors argument.
  • "()" sync groupings: When multiple events need to single in the same frame synchronously, parenthesis are used to group those events. You can group nexted values, a completion or an error in this manner. The position of the initial ( determines the time at which its values are emitted.
  • "^" subscription point: (hot observables only) shows the point at which the tested observables will be subscribed to the hot observable. This is the “zero frame” for that observable, every frame before the ^ will be negative. You can only use this character once in a diagram.
  • "!" unsubscription point: shows the point in time at which a subscription is unsubscribed. You can only use this character once in a diagram.
  • any other character represents a value emitted. The actual value can be represented in the values argument, where the character is the key.


  • hot(marbles: string, values?: object, error?: any) - creates a “hot” observable (a subject) that will behave as though it’s already “running” when the test begins. An interesting difference is that hot marbles allow a ^ character to signal where the “zero frame” is. That is the point at which the subscription to observables being tested begins.
  • cold(marbles: string, values?: object, error?: any) - creates a “cold” observable whose subscription starts when the test begins.
  • expectObservable(actual: Observable<T>).toBe(marbles: string, values?: object, error?: any) - schedules an assertion for when the TestScheduler flushes. The TestScheduler will automatically flush at the end of your jasmine it block.
  • expectSubscriptions(actualSubscriptionLogs: SubscriptionLog[]).toBe(subscriptionMarbles: string) - like expectObservable schedules an assertion for when the testScheduler flushes. Both cold() and hot() return an observable with a property subscriptions of type SubscriptionLog[]. Give subscriptions as parameter to expectSubscriptions to assert whether it matches the subscriptionsMarbles marble diagram given in toBe().


  • hot('--a--b') will emit "a" and "b"
  • hot('--a--b', { a: 1, b: 2 }) will emit 1 and 2
  • hot('---#') will emit error "error"
  • hot('---#', null, new SpecialError('test')) will emit new SpecialError('test')
  • hot('-') equivalent to Observable.never(), or an observable that never emits or completes
  • hot('-a-^-b--|') on frame -20 emit a, then on frame 20 emit b, and on frame 50, complete.
  • hot('-----(a|)') on frame 50, emit a and complete.

Basic test

var e1 = hot('----a--^--b-------c--|');
var e2 = hot(  '---d-^--e---------f-----|');
var expected =      '---(be)----c-f-----|';

  • The ^ characters of hot observables should always be aligned.
  • The first character of cold observables or expected observables should always be aligned with each other, and with the ^ of hot observables.
  • Use default emission values when you can. Specify values when you have to.


© Ellen Langelaar 2018