Angular 4 - TypeError：無法讀取屬性的'長度'null

Question

這是一個基本的Angular 4應用程序，其中Django Rest framework作爲後端。我使用JWT令牌進行身份驗證。當我單擊登錄按鈕時，顯示此錯誤。我找不到爲什麼會發生這種情況。我是新來Typescript和Angular 4，希望有人能解決這個疑難問題都試圖改變signInComponent.html，但問題並沒有解決

** ** SignInComponent.ts

import { NgForm } from '@angular/forms/src/directives'; 
import { Router } from '@angular/router'; 
import { Component, OnInit } from '@angular/core'; 
import { UserService } from '../../services/user.service'; 

@Component({ 
selector: 'app-sign-in', 
templateUrl: './sign-in.component.html', 
styleUrls: ['./sign-in.component.css'] 
}) 
export class SignInComponent { 
token=null ; 
formError: string; 
submitting = false; 

constructor(private router:Router, private authService:UserService) { } 

onSubmit(signInForm: NgForm) { 

if (signInForm.valid) { 

    console.log('submitting...', signInForm); 
    this.submitting = true; 
    this.formError = null; 

    this.authService.login(signInForm.value.username, 
signInForm.value.password) 
    .subscribe((data) => { 
     this.token=data['data']; 
     console.log(this.token['token']) 
     this.authService.saveJWT(this.token['token']); 
     this.authService.isAuthenticated =true; 
     this.router.navigate(['/auth']); 
     }, 
     (err)=> { 
     this.submitting = false; 
     console.log('got error: ', err); 
     this.formError = err; 
     this.router.navigate(['/login']); 
     } 
    ); 

} 
this.authService.ping().subscribe((data) => { 
    console.log('got token yeeh: '); 
    console.log(this.authService.getJwt()); 
    }, 
); 

} 
} 

** ** SignInComponent.html

<div class="sign-in-form"> 
<img src="./assets/logo.jpg" width="320" /> 
<h4>Sign In</h4> 
<form #signInForm="ngForm" (ngSubmit)="onSubmit(signInForm)" novalidate> 
<div class="form-group"> 
<input class="form-control" name="username" required placeholder="User Name" 
ngModel #username="ngModel" /> 
<div [hidden]="username.valid || username.pristine" class="alert alert- 
danger"> 
User Name is required. 
</div> 
</div> 
<div class="form-group"> 
<input class="form-control" name="password" required type="password" 
placeholder="Password" ngModel #password="ngModel"/> 
    <div [hidden]="password.valid || password.pristine" class="alert alert- 
danger"> 
    Password is required. 
    </div> 
</div> 
<div class="checkbox"> 
    <label> 
    <input type="checkbox" name="rememberMe" ngModel /> Remember Me 
    </label> 
</div> 
<div *ngIf="formError" class="alert alert-danger"> 
    {{ formError }} 
</div> 
<div *ngIf="!submitting"> 
    <button type="submit" class="btn btn-primary">Sign In</button> 
</div> 
<div *ngIf="submitting"> 
    <p class="message">Signing In...</p> 
</div> 

</form> 
</div> 

個

** ** AuthService.ts

import { any } from 'codelyzer/util/function'; 
import { Injectable, OnInit } from '@angular/core'; 
import { Router } from '@angular/router'; 
import { HttpClient } from '@angular/common/http'; 
import { Observable } from 'rxjs/Rx'; 
import { HttpHeaders } from '@angular/common/http'; 

@Injectable() 
export class UserService { 


isAuthenticated = false; 

constructor(private router: Router ,private http:HttpClient ) { 

} 

saveJWT(strToken: string): any { 
localStorage.setItem('JWT',strToken); 
} 

login(username: string, password: string):Observable<any> { 

let headers =new HttpHeaders(); 
var body = "username="+username+"&password="+password; 
headers = headers.set("Content-Type", "application/x-www-form-urlencoded"); 
return this.http.post('http://nucore.ddns.net:800/v1/user/login', body, { 
    headers:headers}); 
} 
getJwt():string 
{ 

return localStorage.getItem('JWT'); 

} 
ping():Observable<any> { 
return this.http.get('https://jsonplaceholder.typicode.com/users') 

} 

logOut() { 
// remove user from local storage to log user out 
localStorage.removeItem('JWT'); 
this.isAuthenticated = false; 
this.router.navigate(['/login']); 
} 



} 

網絡截圖

Answer 1

import {Injectable} from '@angular/core'; 
import {Router} from '@angular/router'; 
import {Observable} from 'rxjs'; 
import {Http, Headers} from '@angular/http'; 

@Injectable() 
export class UserService { 


    isAuthenticated = false; 

    constructor(private router: Router, private http: Http) { 

    } 

    saveJWT(strToken: string): any { 
    localStorage.setItem('JWT', strToken); 
    } 

    login(username: string, password: string): Observable<any> { 

    const head = new Headers(); 
    const body = 'username=' + username + '&password=' + password; 
    head.set('Content-Type', 'application/x-www-form-urlencoded'); 
    return this.http.post('http://nucore.ddns.net:800/v1/user/login', body, { 
     headers: head 
    }); 
    } 

    getJwt(): string { 

    return localStorage.getItem('JWT'); 

    } 

    ping(): Observable<any> { 
    return this.http.get('https://jsonplaceholder.typicode.com/users') 

    } 

    logOut() { 
// remove user from local storage to log user out 
    localStorage.removeItem('JWT'); 
    this.isAuthenticated = false; 
    this.router.navigate(['/login']); 
    } 


}