获取未定义的属性

Question

新手问题。下面的代码有什么问题？无法说出get属性TypeError：无法读取未定义的属性'get'

  var http: HttpClient;
  const req = http.get<any>(url)
      .subscribe(
      res => {

Answer 1

您可以这样做

category_id

Answer 2

首先在构造函数中注入HttpClient，然后您可以像这样使用它。

import { Injectable } from '@angular/core';
import { HttpClient, HttpHeaders, HttpParams } from '@angular/common/http';
import { Observable } from 'rxjs/Observable';

@Injectable()
export abstract class DataService {

  constructor(private http: HttpClient) {}

  public get<T>(url: string, params?: any): Observable<T> {
    return this.http.get<T>(url, {headers: headers, params: params});
  }
}

或

const req =this.http.get<any>(url)
      .subscribe(
      res => {