What is the difference between an Observable and a Promise?
Answer
Observables represent streams that can emit multiple values over time. Select the flattening operator from the desired concurrency rule, handle errors in the stream, and ensure subscriptions have a clear lifetime.
For What is the difference between an Observable and a Promise?, start with the rule or behaviour, then anchor it in a small realistic example. Distinguish the default approach from the exceptions and name the observable outcome: correctness, maintainability, accessibility, performance, or security.
Example
results$ = this.query.valueChanges.pipe(
debounceTime(250), distinctUntilChanged(),
switchMap(query => this.api.search(query))
)switchMap cancels the previous inner request when a newer query arrives.
How to structure your answer
- Define the concept in one or two sentences.
- Explain when you would use it and when you would choose an alternative.
- Walk through a small example, including an edge case.
- Close with how you would test or measure the result.
Common mistakes
- Repeating a definition without connecting it to real code.
- Treating an optimization or abstraction as a default rather than a trade-off.
- Omitting lifecycle, error, cleanup, accessibility, or testing considerations when they apply.
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?
Interview tip
For this hard-level question, narrate your assumptions before coding. Interviewers can assess reasoning from a clear, bounded example much better than from a list of APIs.