Argument of type 'typeof MsalService' is not assignable to parameter of type 'MsalService' in angular

3 min read 04-10-2024

Argument of type 'typeof MsalService' is not assignable to parameter of type 'MsalService' in angular

Angular Error: "Argument of type 'typeof MsalService' is not assignable to parameter of type 'MsalService'" Explained

This error often pops up when working with the Microsoft Authentication Library (MSAL) in Angular projects. It indicates a fundamental misunderstanding of how Angular injects dependencies and how to interact with MSAL services.

Understanding the Problem

The error "Argument of type 'typeof MsalService' is not assignable to parameter of type 'MsalService'" arises because you're trying to use the MsalService class itself (the type definition) as a service instance within your component. Angular expects a fully instantiated service object.

Scenario:

Let's imagine you have a component MyComponent that needs to interact with the MSAL service to handle user authentication:

import { Component } from '@angular/core';
import { MsalService } from '@azure/msal-angular';

@Component({
  selector: 'app-my-component',
  templateUrl: './my-component.html'
})
export class MyComponent {

  constructor(private msalService: MsalService) {
    // Error happens here
    this.msalService.loginPopup(); 
  }
}

The Code:

In the above code, we're trying to use msalService.loginPopup() within the component's constructor. However, we're not passing a properly instantiated MsalService object. Instead, we're passing typeof MsalService, which is a reference to the class definition, not an instance.

Why This Error Occurs

Angular's Dependency Injection (DI) system works by injecting pre-configured instances of services into components. When you declare private msalService: MsalService in the constructor, you're telling Angular that you need an instance of the MsalService.

However, by directly using typeof MsalService, you're attempting to use the class definition itself, which hasn't been initialized yet. Angular can't directly use the class definition as a service instance.

Resolving the Error

To resolve this error, you need to ensure that Angular properly injects an instantiated MsalService object into your component. This can be achieved by:

Setting up MSAL Configuration:

In your app.module.ts file, import the MsalModule and provide the MSAL configuration:

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { MsalModule, MsalService, MsalGuard, MsalInterceptor, MsalBroadcastService, MsalRedirectComponent, InteractionType, MSAL_INSTANCE, MSAL_CONFIG, MsalGuardConfiguration } from '@azure/msal-angular';
import { HTTP_INTERCEPTORS, HttpClientModule } from '@angular/common/http';
import { environment } from '../environments/environment';

const isIE = window.navigator.userAgent.indexOf('MSIE ') > -1 || window.navigator.userAgent.indexOf('Trident/') > -1;

@NgModule({
  declarations: [
    // Your components
  ],
  imports: [
    BrowserModule,
    HttpClientModule,
    MsalModule.forRoot(
      {
        clientID: 'your-client-id',
        authority: 'https://login.microsoftonline.com/your-tenant-id',
        redirectUri: environment.redirectUri,
        postLogoutRedirectUri: environment.postLogoutRedirectUri,
        cache: {
          cacheLocation: 'sessionStorage',
          storeAuthStateInCookie: isIE, // Set to true for IE 11
        }
      },
      {
        interactionType: InteractionType.Popup,
        authRequest: {
          scopes: ['user.read']
        }
      },
      {
        popUp: true
      }
    ),
    // ... other imports
  ],
  providers: [
    {
      provide: HTTP_INTERCEPTORS,
      useClass: MsalInterceptor,
      multi: true
    },
    MsalService,
    MsalGuard
  ],
  bootstrap: [
    // Your main component
  ]
})
export class AppModule { }

Injecting the MsalService:

In your component, inject the MsalService in the constructor:

import { Component } from '@angular/core';
import { MsalService } from '@azure/msal-angular'; 

@Component({
  selector: 'app-my-component',
  templateUrl: './my-component.html'
})
export class MyComponent {

  constructor(private msalService: MsalService) {
    // Now you can use the msalService instance
    this.msalService.loginPopup(); 
  }
}

Best Practices

Ensure that the MsalService is properly configured in your app.module.ts.
Inject the MsalService in the constructor of your component.
Refer to the official MSAL for Angular documentation for detailed configuration and usage instructions.
Consider using the MsalGuard to protect routes that require authentication.

Additional Value

By understanding the difference between a class definition and a service instance, and by properly configuring MSAL and injecting the service into your component, you can resolve the "Argument of type 'typeof MsalService' is not assignable to parameter of type 'MsalService'" error and successfully integrate MSAL into your Angular application.

Resources

MSAL for Angular Documentation: https://github.com/AzureAD/microsoft-authentication-library-for-js/blob/dev/lib/msal-angular/docs/v2.x.x/guide/configure.md
MSAL for JavaScript Documentation: https://github.com/AzureAD/microsoft-authentication-library-for-js/blob/dev/lib/msal-js/docs/README.md