Improve types and get rid of @@observable nonsense

README.md

@@ -64,7 +64,13 @@ if (isUnsubscribable(something)) {
 
 ## RxJS compatibility
 
-RxJS' Observable implementation is not standard-conform, especially because it uses a `pipe` method which is not part of the proposed standard. So in order to use Observable instances of this library in RxJS pipes you have to convert the observable with RxJS' `from` function:
+RxJS' Observable implementation is not standard-conform, especially because it uses a `pipe` method which is not part of the proposed standard. RxJS unfortunately also does not use a polyfill for `Symbol.observable`, it just assumes that one is present or otherwise it falls back on using a non-standard `"@@observable"` string instead. To ensure compatibility between RxJS and any other observable implementation make sure to load a `Symbol.observable` polyfill like [symbol-observable] before importing RxJS:
+
+```typescript
+import "symbol-observable";
+```
+
+In order to use Observable instances of this library in RxJS pipes you have to convert the observable with RxJS' `from` function:
 
 ```typescript
 import { from } from "rxjs";

src/main/Observable.ts

@@ -8,6 +8,7 @@ import "symbol-observable";
 import { ObservableLike } from "./ObservableLike.js";
 import { Observer } from "./Observer.js";
 import { isSubscribable, Subscribable } from "./Subscribable.js";
+import type { SubscribeArgs } from "./SubscribeArgs.js";
 import { SubscriberFunction } from "./SubscriberFunction.js";
 import { Subscription } from "./Subscription.js";
 import { SubscriptionImpl } from "./SubscriptionImpl.js";
@@ -41,18 +42,6 @@ export interface ObservableConstructor<T> {
     from(observable: Subscribable<T> | Iterable<T>): ObservableLike<T>;
 }
 
-/**
- * Subscriber arguments.
- */
-export type SubscribeArgs<T = unknown> =
-    | [ Observer<T> ]
-    | [ (value: T) => void, ((error: Error) => void)?, (() => void)? ]
-    | [
-        (((value: T) => void) | null | undefined),
-        (((error: any) => void) | null | undefined)?,
-        ((() => void) | null | undefined)?
-    ];
-
 /**
  * Observable implementation.
  */
@@ -92,14 +81,11 @@ export class Observable<T> implements ObservableLike<T> {
         return createObservableFrom<T>(this, Observable, observable);
     }
 
+    /** @inheritDoc */
     public [Symbol.observable](): this {
         return this;
     }
 
-    public "@@observable"(): this {
-        return this;
-    }
-
     /** @inheritDoc */
     public subscribe(observer: Observer<T>): Subscription;
 
@@ -167,8 +153,7 @@ export function createObservableFrom<T>(thisConstructor: ObservableConstructor<T
             observer.complete();
         });
     } else if (observable instanceof Object) {
-        const observableFactory = (observable as Observable<T>)["@@observable"]
-            ?? (observable as Observable<T>)[Symbol.observable];
+        const observableFactory = (observable as Observable<T>)[Symbol.observable];
         if (observableFactory instanceof Function) {
             const observable = observableFactory();
             if (observable instanceof Object) {

src/main/ObservableLike.ts

@@ -5,6 +5,7 @@
 
 import { Observer } from "./Observer.js";
 import { Subscribable } from "./Subscribable.js";
+import type { SubscribeArgs } from "./SubscribeArgs.js";
 import { Subscription } from "./Subscription.js";
 
 /**
@@ -19,16 +20,10 @@ export interface ObservableLike<T = unknown> extends Subscribable<T> {
         Subscription;
 
     /** @inheritDoc */
-    subscribe(...args: [ Observer<T> ] | [ (value: T) => void, ((error: Error) => void)?, (() => void)? ]):
-        Subscription;
+    subscribe(...args: SubscribeArgs<T>): Subscription;
 
     /**
      * Returns itself.
      */
-    [Symbol.observable]?(): ObservableLike<T>;
-
-    /**
-     * Returns itself. This is used as a fallback for environments which don't support `Symbol.observable`.
-     */
-    "@@observable"?(): ObservableLike<T>;
+    [Symbol.observable](): ObservableLike<T>;
 }

src/main/SubscribeArgs.ts

@@ -0,0 +1,18 @@
+/*
+ * Copyright (C) 2018 Klaus Reimer <[email protected]>
+ * See LICENSE.md for licensing information.
+ */
+
+import type { Observer } from "./Observer.js";
+
+/**
+ * Subscriber arguments.
+ */
+export type SubscribeArgs<T = unknown> =
+    | [ Observer<T> ]
+    | [ (value: T) => void, ((error: Error) => void)?, (() => void)? ]
+    | [
+        (((value: T) => void) | null | undefined),
+        (((error: any) => void) | null | undefined)?,
+        ((() => void) | null | undefined)?
+    ];

src/main/index.ts

@@ -3,11 +3,12 @@
  * See LICENSE.md for licensing information
  */
 
-export { Observable, type SubscribeArgs } from "./Observable.js";
+export { Observable } from "./Observable.js";
 export { type ObservableLike } from "./ObservableLike.js";
 export { type CompleteObserver, type ErrorObserver, type NextObserver, type Observer } from "./Observer.js";
 export { SharedObservable } from "./SharedObservable.js";
 export { isSubscribable, type Subscribable } from "./Subscribable.js";
+export { type SubscribeArgs } from "./SubscribeArgs.js";
 export { type SubscriberFunction } from "./SubscriberFunction.js";
 export { type Subscription } from "./Subscription.js";
 export { type SubscriptionObserver } from "./SubscriptionObserver.js";

src/test/Observable.test.ts

@@ -3,6 +3,8 @@
  * See LICENSE.md for licensing information.
  */
 
+import "symbol-observable";
+
 import { runTests as runObservableTests } from "es-observable-tests";
 import { from, merge, Subject } from "rxjs";
 import { describe, expect, it, vi } from "vitest";

src/test/ObservableLike.test.ts

@@ -0,0 +1,48 @@
+/*
+ * Copyright (C) 2025 Klaus Reimer <[email protected]>
+ * See LICENSE.md for licensing information.
+ */
+
+import "symbol-observable";
+
+import { from, merge, Subject } from "rxjs";
+import { describe, expect, it } from "vitest";
+
+import { Observable } from "../main/Observable.js";
+import type { ObservableLike } from "../main/ObservableLike.js";
+import type { SubscriptionObserver } from "../main/SubscriptionObserver.js";
+
+describe("ObservableLike", () => {
+    it("can be converted into RxJS observable", () => {
+        let exposedObserver: SubscriptionObserver<number> | undefined;
+        const observable: ObservableLike<number> = new Observable<number>(observer => {
+            exposedObserver = observer;
+        });
+        const rxjsObservable = from(observable);
+        const values: number[] = [];
+        rxjsObservable.subscribe(value => values.push(value));
+        if (exposedObserver == null) {
+            throw new Error("Observer not exposed");
+        }
+        exposedObserver.next(1);
+        expect(values).toEqual([ 1 ]);
+        exposedObserver.next(2);
+        expect(values).toEqual([ 1, 2 ]);
+    });
+    it("can be used in RxJS operators", () => {
+        let exposedObserver: SubscriptionObserver<number> | undefined;
+        const observable: ObservableLike<number> = new Observable<number>(observer => {
+            exposedObserver = observer;
+        });
+        const subject = new Subject<number>();
+        const mergedObservable = merge(subject, observable);
+        const values: number[] = [];
+        mergedObservable.subscribe(value => values.push(value));
+        if (exposedObserver == null) {
+            throw new Error("Observer not exposed");
+        }
+        exposedObserver.next(1);
+        subject.next(2);
+        expect(values).toEqual([ 1, 2 ]);
+    });
+});

src/test/SharedObservable.test.ts

@@ -3,6 +3,8 @@
  * See LICENSE.md for licensing information
  */
 
+import "symbol-observable";
+
 import { from } from "rxjs";
 import { describe, expect, it, vi } from "vitest";
 

src/test/index.test.ts

@@ -6,11 +6,12 @@
 import { describe, expect, it } from "vitest";
 
 import * as exports from "../main/index.js";
-import { Observable, type SubscribeArgs } from "../main/Observable.js";
+import { Observable } from "../main/Observable.js";
 import { type ObservableLike } from "../main/ObservableLike.js";
 import { type CompleteObserver, type ErrorObserver, type NextObserver, type Observer } from "../main/Observer.js";
 import { SharedObservable } from "../main/SharedObservable.js";
 import { isSubscribable, type Subscribable } from "../main/Subscribable.js";
+import { type SubscribeArgs } from "../main/SubscribeArgs.js";
 import { type SubscriberFunction } from "../main/SubscriberFunction.js";
 import { type Subscription } from "../main/Subscription.js";
 import { type SubscriptionObserver } from "../main/SubscriptionObserver.js";