rxjs-hook

Subscribe to RxJS Observables and Subjects inside React components. No tearing, concurrent-safe (uses useSyncExternalStore under the hood).

If you know RxJS and React: pick the hook that matches your RxJS type; it returns the latest value (or a callback + stream). No magic.

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Install

pnpm add rxjs-hook react rxjs
# npm install rxjs-hook react rxjs
# yarn add rxjs-hook react rxjs

Peer deps: react ^18 | ^19, rxjs ^7.

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Which hook do I use?

You have…	Use this hook	You get…
BehaviorSubject<T>	useBehaviorSubject	T (always defined)
Subject<T>	useSubject	T or undefined
ReplaySubject<T>	useReplaySubject	T or undefined
AsyncSubject<T>	useAsyncSubject	T or undefined (after complete)
Observable<T>	useObservable	T (you pass initialValue)
Side effects only	useSubscription	nothing (next/error/complete)
Turn callbacks → stream	useObservableCallback	[callback, Subject<T>]

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State hooks (subscribe → current value)

These hooks subscribe to a stream and return the latest value. Component re-renders when the stream emits.

useBehaviorSubject

BehaviorSubject always has a current value. Hook returns it.

import { useBehaviorSubject } from "rxjs-hook";
import { BehaviorSubject } from "rxjs";

const count$ = new BehaviorSubject(0);

function Counter() {
  const count = useBehaviorSubject(count$);
  return <button onClick={() => count$.next(count + 1)}>{count}</button>;
}

useSubject

Subject has no initial value. Hook returns undefined until the first emission, then the latest value.

import { useSubject } from "rxjs-hook";
import { Subject } from "rxjs";

const events$ = new Subject<string>();

function Log() {
  const last = useSubject(events$);
  return <div>{last ?? "—"}</div>;
}

useReplaySubject

ReplaySubject replays N values to new subscribers. Hook returns the latest replayed value (or undefined before any emission).

import { useReplaySubject } from "rxjs-hook";
import { ReplaySubject } from "rxjs";

const state$ = new ReplaySubject<State>(1);

function App() {
  const state = useReplaySubject(state$);
  return state ? <View state={state} /> : null;
}

useAsyncSubject

AsyncSubject emits only when it completes, and only the last value (Promise-like). Hook returns that value after complete(), otherwise undefined.

import { useAsyncSubject } from "rxjs-hook";
import { AsyncSubject } from "rxjs";

const result$ = new AsyncSubject<Data>();
fetch(url).then((r) => r.json()).then((d) => { result$.next(d); result$.complete(); });

function DataView() {
  const data = useAsyncSubject(result$);
  return data ? <pre>{JSON.stringify(data)}</pre> : <span>Loading…</span>;
}

useObservable

Any Observable<T>. You must pass an initialValue; the hook returns it until the first emission, then the latest value.

import { useObservable } from "rxjs-hook";
import { interval, map } from "rxjs";

const ticks$ = interval(1000).pipe(map((n) => n + 1));

function Timer() {
  const tick = useObservable(ticks$, 0);
  return <span>{tick}</span>;
}

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Side effects: useSubscription

Subscribe to an Observable and run next / error / complete. Does not drive React state. Use for logging, analytics, or triggering external logic.

import { useSubscription } from "rxjs-hook";
import { events$ } from "./events";

function Logger() {
  useSubscription(events$, {
    next: (e) => console.log(e),
    error: (err) => console.error(err),
    complete: () => console.log("done"),
  });
  return null;
}

Pass null or undefined as the first argument to “disable” the subscription.

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Events → stream: useObservableCallback

Get a callback and a Subject. Call the callback (e.g. from onChange); values go into the Subject. Subscribe or .pipe() on it (debounce, map, etc.).

import { useObservableCallback, useSubscription } from "rxjs-hook";
import { debounceTime } from "rxjs";

function Search() {
  const [onChange, query$] = useObservableCallback<string>();

  useSubscription(query$.pipe(debounceTime(300)), {
    next: (q) => console.log("search:", q),
  });

  return <input onChange={(e) => onChange(e.target.value)} />;
}

Or combine with useObservable if you want the latest value in state:

const [onChange, query$] = useObservableCallback<string>();
const query = useObservable(query$.pipe(debounceTime(300)), "");
return <input value={query} onChange={(e) => onChange(e.target.value)} />;

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Tree-shaking

Import from subpaths so only the hooks you use are bundled:

import { useBehaviorSubject } from "rxjs-hook/useBehaviorSubject";
import { useObservableCallback } from "rxjs-hook/useObservableCallback";
import { useSubscription } from "rxjs-hook/useSubscription";
// etc.

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Publishing

Push a version tag to run the Publish to npm workflow (lint → test → build → publish):

pnpm version patch && git push && git push origin v0.0.3

Repo secret NPM_TOKEN (npm classic token with Publish scope) is required.

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License

ISC