Support subclassing Observable with non-class constructor functions.

apollographql/apollo-client#7640 (comment)

index.d.ts

@@ -2,3 +2,4 @@ import Observable = require("zen-observable");
 export { Observable };
 export type Observer<T> = ZenObservable.Observer<T>;
 export type Subscription = ZenObservable.Subscription;
+export type Subscriber<T> = ZenObservable.Subscriber<T>;

module.js

@@ -1 +1,29 @@
-export { Observable } from "zen-observable/esm.js";
+import { Observable } from "zen-observable/esm.js";
+
+// When a non-native class constructor function attempts to subclass
+// Observable, compilers like Babel and TypeScript may compile the
+// super(subscriber) expression to Observable.call(this, subscriber) or
+// Observable.apply(this, arguments). Calling a native class constructor
+// in this way is forbidden by the ECMAScript specification, but we can
+// preserve backwards compatibility by overriding the Observable.call and
+// Observable.apply methods with an implementation that calls
+// Reflect.construct instead, when available.
+
+Observable.call = function (instance, subscriber) {
+  // Since Observable.call is a static method, 'this' will typically be the
+  // Observable constructor function, though it could be a subclass of the
+  // Observable constructor, which is why we don't just hard-code it here.
+  return construct(this, instance, subscriber);
+};
+
+Observable.apply = function (instance, args) {
+  return construct(this, instance, args[0]);
+};
+
+function construct(Super, instance, subscriber) {
+  return typeof Reflect === 'object'
+    ? Reflect.construct(Super, [subscriber], instance.constructor)
+    : Function.prototype.call.call(Super, instance, subscriber) || instance;
+}
+
+export { Observable }

tests/Observable.ts

@@ -1,9 +1,80 @@
-const { expect } = require("chai");
+const { expect } = require("chai") as typeof import("chai");
+
+import type {
+  Observable,
+  Subscriber,
+} from "..";
 
 export default function (Observable: typeof import("..").Observable) {
   describe("Observable", () => {
     it("Should be a constructor function", () => {
       expect(typeof Observable).to.equal("function");
     });
+
+    describe('subclassing by non-class constructor functions', () => {
+      function check(constructor: new <T>(sub: Subscriber<T>) => Observable<T>) {
+        constructor.prototype = Object.create(Observable.prototype, {
+          constructor: {
+            value: constructor,
+          },
+        });
+
+        const subscriber: Subscriber<number> = observer => {
+          observer.next(123);
+          observer.complete();
+        };
+
+        const obs = new constructor(subscriber) as Observable<number>;
+
+        expect(typeof (obs as any).sub).to.equal("function");
+        expect((obs as any).sub).to.equal(subscriber);
+
+        expect(obs).to.be.instanceOf(Observable);
+        expect(obs).to.be.instanceOf(constructor);
+        expect(obs.constructor).to.equal(constructor);
+
+        return new Promise((resolve, reject) => {
+          obs.subscribe({
+            next: resolve,
+            error: reject,
+          });
+        }).then(value => {
+          expect(value).to.equal(123);
+        });
+      }
+
+      function newify(
+        constructor: <T>(sub: Subscriber<T>) => void,
+      ): new <T>(sub: Subscriber<T>) => Observable<T> {
+        return constructor as any;
+      }
+
+      it('simulating super(sub) with Observable.call(this, sub)', () => {
+        function SubclassWithSuperCall<T>(sub: Subscriber<T>) {
+          const self = Observable.call(this, sub) || this;
+          self.sub = sub;
+          return self;
+        }
+        return check(newify(SubclassWithSuperCall));
+      });
+
+      it('simulating super(sub) with Observable.apply(this, arguments)', () => {
+        function SubclassWithSuperApplyArgs<T>(_sub: Subscriber<T>) {
+          const self = Observable.apply(this, arguments) || this;
+          self.sub = _sub;
+          return self;
+        }
+        return check(newify(SubclassWithSuperApplyArgs));
+      });
+
+      it('simulating super(sub) with Observable.apply(this, [sub])', () => {
+        function SubclassWithSuperApplyArray<T>(...args: [Subscriber<T>]) {
+          const self = Observable.apply(this, args) || this;
+          self.sub = args[0];
+          return self;
+        }
+        return check(newify(SubclassWithSuperApplyArray));
+      });
+    });
   });
 }