In RxJS, how does a BehaviorSubject differ from a plain Subject, and what does a late subscriber receive from each?

## What a Subject is In RxJS a **Subject** is an object that is both an **Observable** (you can `subscribe` to it) and an **Observer** (you can call `next`, `error` and `complete` on it). Every value passed to `next()` is delivered to every observer subscribed at that moment. It is the simplest way to push values into a stream from imperative code. A plain `Subject` has **no memory**: - A subscriber receives only the values pushed **after** it subscribed. - A value pushed while nobody is subscribed is delivered to nobody and is lost. - There is no way to ask a plain `Subject` "what was the last value?". ## What BehaviorSubject adds `BehaviorSubject<T>` is a `Subject` subclass that models **a value that changes over time** rather than a series of events: 1. Its constructor **requires an initial value**: `new BehaviorSubject<string>('idle')`. 2. Every `next(v)` stores `v` as the current value and then forwards it. 3. When anyone subscribes, it first receives the **current value** synchronously, then every later value. 4. `getValue()` — and the `value` getter, which calls it — returns the current value without subscribing. So a subscriber that arrives late never has to wait for "the next change" to know the state; it receives the state at once. ## Side by side | | `Subject` | `BehaviorSubject` | |---|---|---| | Constructor | no arguments | initial value required | | Late subscriber while active | nothing until the next `next()` | the current value, immediately | | Synchronous read | not available | `getValue()` / `value` | | Late subscriber after `complete()` | completion only | completion only, **no** value | | Late subscriber after `error()` | the error | the error | | `getValue()` after `error()` | — | throws the stored error | ## The edges interviewers probe - **Completed BehaviorSubjects do not replay.** Once `complete()` has been called, a new subscriber gets only the completion notification. The RxJS source documents two differences between `BehaviorSubject` and `new ReplaySubject(1)`: the `BehaviorSubject` comes primed with an initial value, and the `ReplaySubject` keeps replaying after an error where the `BehaviorSubject` does not. The same replay path also runs after completion. - **`getValue()` after completion still works.** The last value remains readable; only after `error()` does `getValue()` throw — it rethrows the error the subject was errored with. - **Calling `unsubscribe()` on the subject itself** (not on a subscription) closes it for good: later `next()`, `subscribe()` and `getValue()` calls throw an `ObjectUnsubscribedError`. To stop listening, unsubscribe the `Subscription`, not the subject. - **Reading `getValue()` everywhere is a smell.** It is a synchronous escape hatch; code that reads it inside other streams usually wants to combine the subject as a stream instead, so it reacts to changes. ```ts import { BehaviorSubject, Subject } from 'rxjs'; const events = new Subject<string>(); const status = new BehaviorSubject<string>('idle'); events.next('clicked'); // nobody subscribed: lost status.next('loading'); // stored as the current value events.subscribe((v) => console.log('events', v)); // logs nothing yet status.subscribe((v) => console.log('status', v)); // status loading console.log(status.getValue()); // 'loading' status.complete(); status.subscribe({ next: (v) => console.log('late', v), complete: () => console.log('late complete'), }); // late complete (no value replayed after completion) console.log(status.value); // 'loading' is still readable ``` ## Choosing between them - Use a **`Subject`** for **events** where only future occurrences matter: a button was clicked, a message arrived, a refresh was requested. Replaying an old event to a new subscriber would be wrong. - Use a **`BehaviorSubject`** for **state** that always has a current value: the selected tab, the signed-in user (or `null`), a filter. Every consumer needs the current value on arrival. - If there is no sensible initial value, a `ReplaySubject(1)` gives "latest value to late subscribers" without inventing a seed — at the cost of having no synchronous `getValue()`.