State with a BehaviorSubject in RxJS

RxJS interview exercise on Subjects & sharing, difficulty 2 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.

Write Store<T>: constructed with an initial state; `state emits the current state to every subscriber at once and…" /> emits the current state to every subscriber at once and…" /> emits the current state to every subscriber at once and…" /> emits the current state to every subscriber at once and…" /> emits the current state to every subscriber at once and…" /> emits the current state to every subscriber at once and…" /> emits the current state to every subscriber at once and…" /> emits the current state to every subscriber at once and then every change; get() returns the current state; update(fn) replaces it with fn(current); select(key) emits state[key] now and whenever it changes, skipping emissions where it did not change.

Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.

More Subjects & sharing in RxJS

• A Subject as an event bus
• The last few, for latecomers
• One source, many subscribers
• A custom operator

All RxJS exercises · Other languages and frameworks