40. ReplaySubject: a history of values

ReplaySubject — a history buffer

ReplaySubject(bufferSize) stores the last N values. Every new subscriber receives them by "replay" — like a pre-recorded episode.

The three Subject types compared

Type	What it remembers	Initial needed?
`Subject`	nothing	no
`BehaviorSubject`	the 1 latest	yes
`ReplaySubject(N)`	the N latest	no

Where it's used

Analytics: a "history of user actions."
A cache of the latest values that a new subscriber can inspect.
Handing "already-happened" events to a late subscription.

Your task

Create const history$ = new ReplaySubject(2); — a buffer of 2.
Before subscribing, send 1, 2, 3 in turn via history$.next(...).
After subscribing, send 4.
Expected output: --- Subscribe --- → Value: 2 → Value: 3 → Value: 4. The value 1 dropped out of the buffer (it holds only the last 2).

Solution spoiler · click to reveal

const { ReplaySubject } = Rx;

const history$ = new ReplaySubject(2);

history$.next(1);
history$.next(2);
history$.next(3);

console.log('--- Subscribe ---');
history$.subscribe(value => console.log('Value: ' + value));

history$.next(4);

script.ts // TypeScript

CONSOLE · Console output

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