47. Marble testing: your first stream test

Why marble tests

Until now we've tested streams with real timers or with flush(). Marble tests go further: they describe visually, as a string, what a stream should do and when. It's both more readable and more reliable.

Marble syntax

Each character in the string = one "frame" of virtual time (1ms by default).

• - — an empty frame (a step of time).
• a letter (a, b, c...) — a value in that frame (the value map is passed as a separate argument).
• | — completion (complete).
• # — an error (error).
• () — a group of events in one frame.
• 10ms — an explicit duration.

'a-b-|' = value a, pause, value b, pause, complete

Glossary

• cold(marble, values) — creates a cold Observable.
• expectObservable(stream$).toBe(marble, values) — asserts the stream matches the marble string.

const values = { a: 'A', b: 'B' };
const source$ = cold('a-b-|', values);

expectObservable(source$).toBe('a-b-|', values);
assertionWritten = true;