TypeError: Cannot read property '_id' of undefined angular7

Question

我有一个组件员工我正在尝试创建一个表单，为此我创建了 "employee component"、"employee service" 和 "employee model"

在 employee.component.html 我有下面的表格

 <form #employeeform="ngForm" (ngSubmit)="onSubmit(employeeform)">
          <input type="hidden" name="_id"  [(ngModel)]="employeeservice.selectedEmployee._id" #_id="ngModel"/>
        </form>

在employee.component.ts中我有以下代码

 import { Component, OnInit } from '@angular/core';
 import { EmployeeService } from '../shared/employee.service';

 @Component({
    selector: 'app-employee',
    templateUrl: './employee.component.html',
    styleUrls: ['./employee.component.css'],
    providers : [EmployeeService]
 });

 export class EmployeeComponent implements OnInit {
    constructor(private employeeservice : EmployeeService) { }
      ngOnInit() {
      }
   }

这是我在员工组件中导入的employee.service.ts文件

 import { Injectable } from '@angular/core';
 import { HttpClient } from '@angular/common/http';
 import { Observable } from 'rxjs';
 import { Employee } from './employee.model';

 @Injectable({
   providedIn: 'root'
 })
 export class EmployeeService {
   selectedEmployee : Employee;
   employees: Employee[];

   constructor() { }
 }

这是我在员工服务

中导入的employee.model.ts

 export class Employee {
     _id: string;
     name: string;
     position : string;
     office : string;
     salary : number;
 }

当我运行命令 "ng serve"

时，我的浏览器控制台出现以下错误

 EmployeeComponent.html:9 ERROR TypeError: Cannot read property '_id' of undefined
at Object.eval [as updateDirectives] (EmployeeComponent.html:9)
at Object.debugUpdateDirectives [as updateDirectives] (core.js:23911)
at checkAndUpdateView (core.js:23307)
at callViewAction (core.js:23548)
at execComponentViewsAction (core.js:23490)
at checkAndUpdateView (core.js:23313)
at callViewAction (core.js:23548)
at execComponentViewsAction (core.js:23490)
at checkAndUpdateView (core.js:23313)

我是否遗漏了表单需要的任何导入或可能是什么错误。我是初学者，正在努力学习 angular。我用谷歌搜索了解决方案，但找不到任何相关的解决方案。请帮帮我

Answer 1

我建议您在 EmployeeComponent 中创建一个员工实例，并且每次打开该组件时 - 从服务中检索它并将其定位到 html 代码。类似于：

export class EmployeeComponent implements OnInit {
    employee: Employee;

    constructor(private employeeservice : EmployeeService) { 
      this.employee = employeeservice.getSelectedEmployee();
    }
      ngOnInit() {
      }
   }

之后在 html:

 <form #employeeform="ngForm" (ngSubmit)="onSubmit(employeeform)">
          <input type="hidden" name="_id"  [(ngModel)]="employee._id" #_id="ngModel"/>
        </form>

或者用Injection再传一遍。目前，问题是您的 EmployeeComponent 无法像您希望的那样定位（从 html 传递到服务变量），因此您得到 - Undefined 错误。

Answer 2

最初，加载组件时，employee.service 没有为 selectedEmployee 设置值。 html 被呈现，[(ngModel)]="employeeservice.selectedEmployee._id" 期望值为 employeeservice.selectedEmployee._id。但是 employeeservice.selectedEmployee 尚未初始化，因此出现错误。

我建议您使用默认的 _id，以防您没有设置 employeeservice.selectedEmployee._id。

为此使用 [(ngModel)]="employeeservice.selectedEmployee ? employeeservice.selectedEmployee._id : 0"

Answer 3

正如错误所说：你想访问 _id 但 selectedEmployee 从未真正定义过。

您可以只显示在 html 元素上使用 *ngIf 定义的元素，或者给它一个默认值，例如 selectedEmployee : Employee = {}（设置任何需要的变量class 相应地）

Answer 4

你的employee.service.ts文件应该是这样的。

 import { Injectable } from '@angular/core';
 import { HttpClient } from '@angular/common/http';
 import { Observable } from 'rxjs';
 import { Employee } from './employee.model';

 @Injectable({
   providedIn: 'root'
 })
 export class EmployeeService {
     selectedEmployee: Employee = {    
    _id: '',
    name: '',
    position: '',
    office: '',
    salary: null
  };
   employees: Employee[];

   constructor() { }
 }

TypeError: Cannot read property '_id' of undefined angular7

TypeError: Cannot read property '_id' of undefined angular7

angular

angular7

angular-services

angular-ngmodel

angular-forms