When do you use BehaviorSubject?

Answer

Use a BehaviorSubject when subscribers need the current value immediately, such as a small state store. It requires an initial value and exposes the latest value, so avoid it for one-off event streams.

Context

RxJS models asynchronous values as composable streams. The important design choice is the source lifetime and concurrency rule: cancel, merge, queue, or ignore competing work deliberately.

Example

const results$ = query$.pipe(
  debounceTime(250),
  distinctUntilChanged(),
  switchMap(query => api.search(query))
)

switchMap ensures an older search result cannot overwrite a newer query.

Practical considerations

1. Choose the approach from the requirement and constraints, not from habit.
2. Include validation, error handling, and cleanup where the boundary requires them.
3. Verify the observable result with focused tests or measurement.

Follow-up prompts

• What failure mode would you expect if this were implemented incorrectly?
• How would you test this behaviour?
• What changes when the feature must scale to a larger application or team?

In practice

For this medium-level topic, make assumptions explicit, choose the smallest safe implementation, and verify the behavior at the relevant boundary.