feat(isObservable): a new method for checking to see if an object is …

…an RxJS Observable

I've seen a lot of code that checks to see if something is an observable by doing an `typeof` or even `instanceOf` check. It's plausible that in the future these tests might break, as they're testing implementation details of the library. It is recommended that people use this `isObservable` method to see if an object is a compatible RxJS Observable.

spec/util/isObservable-spec.ts

@@ -0,0 +1,26 @@
+import { Observable, isObservable } from 'rxjs';
+import { expect } from 'chai';
+
+describe('isObservable', () => {
+  it('should return true for RxJS Observable', () => {
+    const o = new Observable<any>();
+    expect(isObservable(o)).to.be.true;
+  });
+
+  it('should return true for an observable that comes from another RxJS 5+ library', () => {
+    const o: any = {
+      lift() { /* noop */ },
+      subscribe() { /* noop */ },
+    };
+
+    expect(isObservable(o)).to.be.true;
+  });
+
+  it('should NOT return true for any old subscribable', () => {
+    const o: any = {
+      subscribe() { /* noop */ },
+    };
+
+    expect(isObservable(o)).to.be.false;
+  });
+});

src/index.ts

@@ -30,6 +30,7 @@ export { Notification } from './internal/Notification';
 export { pipe } from './internal/util/pipe';
 export { noop } from './internal/util/noop';
 export { identity } from './internal/util/identity';
+export { isObservable } from './internal/util/isObservable';
 
 /* Error types */
 export { ArgumentOutOfRangeError } from './internal/util/ArgumentOutOfRangeError';

src/internal/util/isObservable.ts

@@ -0,0 +1,10 @@
+import { Observable } from '../Observable';
+import { ObservableInput } from '../types';
+
+/**
+ * Tests to see if the object is an RxJS {@link Observable}
+ * @param obj the object to test
+ */
+export function isObservable<T>(obj: any): obj is Observable<T> {
+  return obj && obj instanceof Observable || (typeof obj.lift === 'function' && typeof obj.subscribe === 'function');
+}