fix(errors): Better error message when you return non-observable thin…

…gs, (#2152)

like `undefined`

fixes #215

spec/util/subscribeToResult-spec.ts

@@ -125,7 +125,8 @@ describe('subscribeToResult', () => {
     const subscriber = new OuterSubscriber(x => {
       done(new Error('should not be called'));
     }, (x) => {
-      expect(x).to.be.an('error', 'invalid observable');
+      expect(x).to.be.an('error');
+      expect(x.message).to.be.equal('Provided object does not correctly implement Symbol.observable');
       done();
     }, () => {
       done(new Error('should not be called'));
@@ -138,12 +139,31 @@ describe('subscribeToResult', () => {
     const subscriber = new OuterSubscriber(x => {
       done(new Error('should not be called'));
     }, (x) => {
-      expect(x).to.be.an('error', 'unknown type returned');
+      expect(x).to.be.an('error');
+      const msg = 'You provided an invalid object where a stream was expected.'
+        + ' You can provide an Observable, Promise, Array, or Iterable.';
+      expect(x.message).to.be.equal(msg);
       done();
     }, () => {
       done(new Error('should not be called'));
     });
 
     subscribeToResult(subscriber, {});
   });
+
+  it('should emit an error when trying to subscribe to a non-object', (done: MochaDone) => {
+    const subscriber = new OuterSubscriber(x => {
+      done(new Error('should not be called'));
+    }, (x) => {
+      expect(x).to.be.an('error');
+      const msg = 'You provided \'null\' where a stream was expected.'
+        + ' You can provide an Observable, Promise, Array, or Iterable.';
+      expect(x.message).to.be.equal(msg);
+      done();
+    }, () => {
+      done(new Error('should not be called'));
+    });
+
+    subscribeToResult(subscriber, null);
+  });
 });

src/util/subscribeToResult.ts

@@ -1,6 +1,7 @@
 import { root } from './root';
 import { isArray } from './isArray';
 import { isPromise } from './isPromise';
+import { isObject } from './isObject';
 import { Subscriber } from '../Subscriber';
 import { Observable, ObservableInput } from '../Observable';
 import { $$iterator } from '../symbol/iterator';
@@ -31,9 +32,7 @@ export function subscribeToResult<T>(outerSubscriber: OuterSubscriber<any, any>,
     } else {
       return result.subscribe(destination);
     }
-  }
-
-  if (isArray(result)) {
+  } else if (isArray(result)) {
     for (let i = 0, len = result.length; i < len && !destination.closed; i++) {
       destination.next(result[i]);
     }
@@ -55,7 +54,7 @@ export function subscribeToResult<T>(outerSubscriber: OuterSubscriber<any, any>,
       root.setTimeout(() => { throw err; });
     });
     return destination;
-  } else if (typeof result[$$iterator] === 'function') {
+  } else if (result && typeof result[$$iterator] === 'function') {
     const iterator = <any>result[$$iterator]();
     do {
       let item = iterator.next();
@@ -68,15 +67,18 @@ export function subscribeToResult<T>(outerSubscriber: OuterSubscriber<any, any>,
         break;
       }
     } while (true);
-  } else if (typeof result[$$observable] === 'function') {
+  } else if (result && typeof result[$$observable] === 'function') {
     const obs = result[$$observable]();
     if (typeof obs.subscribe !== 'function') {
-      destination.error(new Error('invalid observable'));
+      destination.error(new TypeError('Provided object does not correctly implement Symbol.observable'));
     } else {
       return obs.subscribe(new InnerSubscriber(outerSubscriber, outerValue, outerIndex));
     }
   } else {
-    destination.error(new TypeError('unknown type returned'));
+    const value = isObject(result) ? 'an invalid object' : `'${result}'`;
+    const msg = `You provided ${value} where a stream was expected.`
+      + ' You can provide an Observable, Promise, Array, or Iterable.';
+    destination.error(new TypeError(msg));
   }
   return null;
 }