如何解决此＆＃x27; string | null＆＃x27;无法分配给类型的参数＆＃x27; string＆＃x27; .type＆＃x27; null＆＃x27;不能分配给类型＆＃x27; String＆＃x27;

Question

//我在网上购物我被此登录服务所困扰

import { Component, OnInit } from '@angular/core';
import { UserService } from '../user.service';
import { Router } from '@angular/router';
import { EmployeeService } from '../employee.service';
@Component({
  selector: 'app-login',
  templateUrl: './login.component.html',
  styleUrls: ['./login.component.css']
})
export class LoginComponent implements OnInit {

  login = true;
  attemptsLeft = 3;
  userID!:number;
  isAdmin:boolean = false;
  isLoggedIn:boolean = false;
  constructor(public _service:UserService, public userSer:UserService, public route: Router, public empSer: EmployeeService) { }

  ngOnInit(): void {

/* 类型'字符串|的参数| null'不能分配给类型“字符串”的参数。类型'null'不能分配给类型'string'

this.userID = JSON.parse((localStorage.getItem("userId")));//problem here 

    if(this.userID.toString() != ""){
      this.isLoggedIn = true;
    }
    let employee = this.empSer.getEmployee(this.userID).subscribe(employee =>{
      this.isAdmin = employee[0].isAdmin;
    });
    //if user -> send to user
    if(this.isLoggedIn){
      this.route.navigate(['/home-page']);
    }
  }
  checkLogin(loginRef:any){
    this._service.attemptLogin(loginRef).
    subscribe((id: string)=>{
      console.log(id);
      if(id == ""){
        console.log("incorrect");
        this.login = false;

问题类型'string | null'不能分配给类型“字符串”的参数。类型'null'不能分配给类型'string'

    let incorrectAttempts = 
 JSON.parse(sessionStorage.getItem("incorrectAttempts"));
        console.log(incorrectAttempts);
        if(incorrectAttempts == null){
          incorrectAttempts = 0;
        }
        incorrectAttempts++;
        this.attemptsLeft --;

        if(incorrectAttempts > 2){
          this.userSer.lockUser(loginRef);
        }

问题类型'string | null'不能分配给类型“字符串”的参数。键入“ null”不可分配给type'string'

    sessionStorage.setItem("incorrectAttempts", `JSON.stringify(incorrectAttempts));`
      }
      else{
        localStorage.setItem("userId",JSON.stringify(id));
        sessionStorage.setItem("incorrectAttempts","0");
        this.route.navigate(['/']);
      }
    },(error: any)=>console.log(error));
    
  }
}
//user Controller  this is backend service for login 
let login = (req, res) => {
    let email = req.body.emailId;
    let password = req.body.password;
    UserModel.findOne({emailId:email},(err,user) => {
        console.log("looking for user");
        try{
            if(user.password != password){
                res.send(null);
                return;
            }
        }catch(err){
            console.log(err);
        }

        try{
            res.send(user._id.toString());
            return;
        }catch(err){}
    })

    EmployeeModel.findOne({email:email},(err,employee) => {
        console.log("looking for employee");
        console.log(employee);
        try{
            if(employee.password != password){
                res.send(null);
                return;
            }
        }catch(err){
            return;
        }
        res.send(employee._id.toString());
    })
}
//server.js login 
baseURL = "http://localhost:8080/user"
attemptLogin(loginRef:any):any{
  return this._http.post(`${this.baseURL}/login`,loginRef,{responseType:"text"})
}
please help me ! Pictures [This is the Error Picture][1]

Answer 1

您不一定会从localstorage.getItem（“ userId”）中返回可以解析的任何内容。

如果UserID只是一个字符串，它似乎是

this.userID = localStorage.getItem("userId"));//problem here 
this.isLoggedIn = this.userID != "";

一个字符串 。

，则可以将其返回而无需json。在这种情况下，您可以进行以下操作

let idString: string = localStorage.getItem("userId");
this.userID = idString ? JSON.parse(idString) : null; //problem was here
this.loggIn = this.userID != null;

您似乎在其他示例中遇到了相同的问题，但是您可以以相同的方式修复它们，除非

sessionStorage.setItem("incorrectAttempts", `JSON.stringify(incorrectAttempts));`

应该是

 sessionStorage.setItem("incorrectAttempts", JSON.stringify(incorrectAttempts));