// src/app/app.component.ts
...
ngOnInit () {
   const headers: Headers = new Headers();
   headers.append('Accept','application/json');
   headers.append('Content-Type','application/json');
   headers.append('Access-Control-Allow-Origin','*');

   const options = new RequestOptions({
     headers: headers
   });

   this.http.get('http://localhost:9000/api/v1/users',options)
    .map((res: Response) => res.json())
    .subscribe(users => console.log(users));
}

…

当应用程序启动时,OnInit挂钩被调用,我在控制台中出错.这是我收到的错误消息.

EXCEPTION: TypeError: Cannot read property 'toString' of null
browser_adapter.js:84EXCEPTION: TypeError: Cannot read property 'toString' of nullBrowserDomAdapter.logError @ browser_adapter.js:84BrowserDomAdapter.logGroup @ browser_adapter.js:94ExceptionHandler.call @ exception_handler.js:65(anonymous function) @ application_ref.js:267ZoneDelegate.invoke @ zone.js:323onInvoke @ ng_zone_impl.js:53ZoneDelegate.invoke @ zone.js:322Zone.run @      zone.js:216(anonymous function) @ zone.js:571ZoneDelegate.invokeTask @ zone.js:356onInvokeTask @ ng_zone_impl.js:44ZoneDelegate.invokeTask @ zone.js:355Zone.runTask @ zone.js:256drainMicroTaskQueue @ zone.js:474
zone.js:461 Unhandled Promise rejection: Cannot read property 'toString' of null ; Zone: <root> ; Task: Promise.then ; Value: TypeError: Cannot read property 'toString' of null(…)consoleError @ zone.js:461_loop_1 @ zone.js:490drainMicroTaskQueue @ zone.js:494
zone.js:463 Error: Uncaught (in promise): TypeError: Cannot read property 'toString' of null(…)

如果我完全删除选项和标题,我从服务器得到CORS错误但响应状态为200. (我想如果我可以传递正确的标题,它应该可以工作.)

this.http.get('http://localhost:9000/api/v1/users')
  .map((res: Response) => res.json())
  .subscribe(users => console.log(users));

这是控制台的错误

XMLHttpRequest cannot load http://localhost:4000/api/v1/users. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:4200' is therefore not allowed access.
browser_adapter.js:93EXCEPTION: Response with status: 0  for URL: null
browser_adapter.js:84EXCEPTION: Response with status: 0  for URL: nullBrowserDomAdapter.logError @         browser_adapter.js:84BrowserDomAdapter.logGroup @    browser_adapter.js:94ExceptionHandler.call @ exception_handler.js:65next @ application_ref.js:348schedulerFn @ async.js:89SafeSubscriber.__tryOrUnsub @ Subscriber.js:225SafeSubscriber.next @ Subscriber.js:174Subscriber._next @ Subscriber.js:124Subscriber.next @ Subscriber.js:88Subject._finalNext @     Subject.js:128Subject._next @ Subject.js:120Subject.next @ Subject.js:77EventEmitter.emit @ async.js:77onHandleError @ ng_zone_impl.js:74ZoneDelegate.handleError @ zone.js:327Zone.runTask @ zone.js:259ZoneTask.invoke @ zone.js:423
Subscriber.js:229Uncaught Response with status: 0  for URL: null

有人可以帮帮忙吗？谢谢.