Angular4注入服务类-调用服务方法时构造函数中的代码未完成？

我正在尝试进入Angular4 / 5。我已连接到名为Sanity（https://www.sanity.io/）的CMS服务。我遇到的问题是，此服务被注入到ProductsComponent类，并且我在组件的ngOnInit方法中调用了该服务中的方法。但是，当我尝试使用ng serve命令重新编译时，出现以下错误：

ERROR in src/app/sanity.service.ts(21,14): error TS2339: Property 'fetch' does not exist on type 'object'.

这是我注入服务的组件：

import { Component, OnInit } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Query } from '@angular/core/src/metadata/di';
import { SanityService } from '../sanity.service';

@Component({
    selector: 'app-product',
    templateUrl: './products.component.html',
    styleUrls: ['./products.component.css']
})
export class ProductsComponent implements OnInit {

    products: object[];

    constructor(private http: HttpClient, private sanityService: SanityService) { }

    ngOnInit() {

        this.sanityService.getProducts((res) => {
            this.products = res;
        });

    }

}

这是服务代码（sanity.service.ts）：

import { Injectable } from '@angular/core';
//const sanityClient = require('@sanity/client');
import * as SanityClient from '@sanity/client';

@Injectable()
export class SanityService {

    sanityClient: object;

    constructor() {
        this.sanityClient = SanityClient({
            projectId: 'someidhere',
            dataset: 'somedatabase',
            useCdn: true
        });
    }

    getProducts(callback) {
        let query = "*[_type == 'product']{ name, _id, description, price, 'imageUrl': image.asset->url }";
        this.sanityClient
            .fetch(
                query, // Query
                {} // Params (optional)
            ).then(res => {
                //console.log('5 movies: ', res)
                //this.products = res;
                callback(res);
            }).catch(err => {
                console.error('Oh no, error occured: ', err)
            });
    }



    getProductById(id, callback) {
        var query = `*[_id == '${id}']{ name, _id, description, price, 'imageUrl': image.asset->url }`;
        this.sanityClient
            .fetch(
                query, // Query
                {} // Params (optional)
            ).then(res => {
                //console.log('PRODUCT: ', res)
                //this.products = res;
                callback(res);
            }).catch(err => {
                console.error('Oh no, error occured: ', err)
            });
    }

}

我的怀疑是，在调用“ getProducts（）”函数时，在服务的构造函数中创建/设置的对象尚未完成“加载”，因此，“ fetch”属性不存在错误。

我真的很难过...有人可以帮我吗？

另外，这是我的app.component.ts，以防万一：

import { Component } from '@angular/core';
import { SanityService } from './sanity.service';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
  providers: [SanityService]
})
export class AppComponent {
  title = 'app';

  constructor(private sanityService: SanityService) {}
}