api – angular2 http.get()不向服务器发送请求

我可以在我的api服务器上看到没有得到请求.这导致了错误. api工作正常我试着做一个卷曲,我得到了预期的结果.

EXCEPTION: Uncaught (in promise): TypeError: Cannot read property ‘subscribe’ of undefined

api.service

import { Injectable } from '@angular/core';
import { Headers,Http,Response,URLSearchParams,RequestOptions } from '@angular/http';
import { Observable } from 'rxjs/Rx';

@Injectable()
export class ApiService {
    headers: Headers = new Headers({
        'Accept': 'application/json','Content-Type': 'application/json'
    });

    api_url: string = 'http://localhost:5001';

    constructor(private http: Http) { }

    private getJson(response: Response) {
        return response.json().results;
    }

    checkForError(response: Response): Response {
        if (response.status >= 200 && response.status < 300) {
            return response;
        } else {
            var error = new Error(response.statusText);
            error['response'] = response;
            Observable.throw(error);
        }
    }

    get(path: string,params: URLSearchParams): Observable<any> {
        return this.http
            .get(`${this.api_url}${path}/`,{ headers: this.headers })
            .map(this.checkForError)
            .catch(err => Observable.throw(err))
            .map(this.getJson);
    }

api.component.ts

import { Component,OnInit } from "@angular/core";
import { ApiService } from './api.service';

@Component({
    selector: 'item',templateUrl: './item.component.html'
})
export class ItemComponent implements OnInit {
    private itemData;
    private errorAlert;

    constructor(private apiService: ApiService) {}

      ngOnInit() {
          this.apiService.get('/items').subscribe(
              result => this.itemData = result,error => this.errorAlert = {msg: error,type: 'danger',closable: true},);
      }
}

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