fix(subscriber): makes sure default callbacks are provided

default callbacks should be provided both - when given a subscriber object and also when given
separate callback functions. Default error callback should re-throw the error

BREAKING CHANGE: default callbacks are now provided both - when given a subscriber object and also
when given separate callback functions. Also default error callback now re-throws the error instead
of th rowing a new Error()

src/index.test.js

@@ -54,7 +54,39 @@ describe('subscribe', () => {
 
 	  it("should work as a standalone function", (done) =>
 	    of(1) |> subscribe({next: () => done()}))
+
+    it("should throw if next callback is not defined", (done) => {
+      try {
+        of(1).subscribe({})
+      } catch (ex) {
+        done()
+      }
+    })
+
+    it("should provide a default complete callback if it is not provided", (done) => {
+      const error = chai.spy()
+      try {
+        of(1).subscribe({
+          next: nx,
+          error: err
+        })
+      } catch (ex) {
+        error(ex)
+      }
+      expect(error).to.not.have.been.called()
+      done()
+    })
+
+    it("should provide a default error callback that re-throws an error if error callback is not provided", (done) => {
+      try {
+        error('bad').subscribe({next: nx})
+      } catch (ex) {
+        expect(ex).to.equal('bad')
+        done()
+      }
+    })
   })
+
   describe('called with callback functions', () => {
     it("should call next", (done) =>
       of(1).subscribe(() => done()))
@@ -64,7 +96,36 @@ describe('subscribe', () => {
 
     it("should call complete", (done) =>
       of(1).subscribe(nx, err, _ => done()))
+
+    it("should throw if next callback is not defined", (done) => {
+      try {
+        of(1).subscribe()
+      } catch (ex) {
+        done()
+      }
+    })
+
+    it("should provide a default complete callback if it is not provided", (done) => {
+      const error = chai.spy()
+      try {
+        of(1).subscribe(nx, err)
+      } catch (ex) {
+        error(ex)
+      }
+      expect(error).to.not.have.been.called()
+      done()
+    })
+
+    it("should provide a default error callback that re-throws an error if error callback is not provided", (done) => {
+      try {
+        error('bad').subscribe(nx)
+      } catch (ex) {
+        expect(ex).to.equal('bad')
+        done()
+      }
+    })
   })
+
   describe('unsubscribe', () => {
     it("should return a subscription object", (done) => {
       const subscription = of(1).subscribe(() => done())

src/internal/subscriber.js

@@ -1,8 +1,13 @@
-export const Subscriber = (nextOrSubscriber, error, complete) =>
-  typeof nextOrSubscriber === 'object'
-    ? nextOrSubscriber
-    : {
-      next: nextOrSubscriber,
-      error: error || (() => {throw new Error()}),
-      complete: complete || (() => {})
-    }
+export const Subscriber = (nextOrSubscriber, error, complete) => {
+  if (typeof nextOrSubscriber === 'object') {
+    nextOrSubscriber.error = nextOrSubscriber.error  || (err => {throw err})
+    nextOrSubscriber.complete = nextOrSubscriber.complete  || (() => {})
+    return nextOrSubscriber
+  }
+
+  return {
+    next: nextOrSubscriber,
+    error: error || (err => {throw err}),
+    complete: complete || (() => {})
+  }
+}