angular6 httpclient已經(jīng)為字段賦值了，為什么字段一直undefined？

已經(jīng)執(zhí)行到黃色部分了，res['data'].itemMaterialType=3，
this.itemMaterialType = res['data'].itemMaterialType; 這句話已經(jīng)執(zhí)行了，為什么this.itemMaterialType一直是undefined;理論上this.itemMaterialType=3才對(duì)啊;

以下是封裝的方法，

  async searchData() {
    this.itemMaterialType = '4';
    await this.api.Dispatch.queryProcessDetailByProcessId({
      processId: this.processId
    })
      .then(res => {
        this.data = res['data'];
        this.isFeedMaterial = res['data'].isFeedMaterial.toString();
        this.itemMaterialType = res['data'].itemMaterialType;
      });
  }

    queryProcessDetailByProcessId: async (option) => await this.http.getByPromise('/api/ScheduleProcess/GetProcessDetail', option),

  async getByPromise(url: string, params: {}) {
    return await this.http.get(this.domain + url, { observe: 'body', params: params })
      .toPromise();
  }