[fixes #336] finally must not fail for non-callable callback

lib/es6-promise/promise.js

@@ -410,8 +410,12 @@ class Promise {
     let promise = this;
     let constructor = promise.constructor;
 
-    return promise.then(value => constructor.resolve(callback()).then(() => value),
-                       reason => constructor.resolve(callback()).then(() => { throw reason; }));
+    if ( isFunction(callback) ) {
+      return promise.then(value => constructor.resolve(callback()).then(() => value),
+                         reason => constructor.resolve(callback()).then(() => { throw reason; }));
+    }
+
+    return promise.then(callback, callback);
   }
 }
 

test/extension-test.js

@@ -1258,6 +1258,26 @@ describe('Promise.prototype.finally', function() {
           done();
         });
       });
+
+      it("preserves the original fulfillment value even if a non-callable callback is given", function(done) {
+        var fulfillmentValue = 1;
+        var promise = Promise.resolve(fulfillmentValue);
+
+        promise['finally']().then(function(value) {
+          assert.equal(fulfillmentValue, value);
+          done();
+        });
+      });
+
+      it("preserves the original rejection reason even if a non-callable callback is given", function(done) {
+        var rejectionReason = new Error();
+        var promise = Promise.reject(rejectionReason);
+
+        promise['finally']().then(undefined, function(reason) {
+          assert.equal(rejectionReason, reason);
+          done();
+        });
+      });
     });
 
     describe("exception cases do propogate the failure", function(){