Скачать или смотреть Resolving TypeError: Cannot read property 'subscribe' of undefined in Angular 9

Learn how to fix the `TypeError: Cannot read property 'subscribe' of undefined` error in Angular 9, explore solutions using Observables effectively, and ensure proper code execution order for user session validation.
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Resolving TypeError: Cannot read property 'subscribe' of undefined in Angular 9

If you're developing an Angular 9 application and trying to manage user sessions, you might run into a frustrating error: TypeError: Cannot read property 'subscribe' of undefined. This error typically indicates that something is amiss in the way your observables are set up or called in your code. In this post, we will walk through the problem and how to resolve it effectively.

Understanding the Problem

The goal is to check if a user session exists when an application component is initialized. If the session is found, the user should be logged in automatically. However, the error you're encountering suggests that the observable you're trying to subscribe to is undefined.

In your case, you were attempting to subscribe to the isLoggedIn getter in your Sidebar.component.ts like this:

[[See Video to Reveal this Text or Code Snippet]]

The issue arises because your isLoggedIn getter in auth.service.ts is not returning an observable, leading to the TypeError when you attempt to call .subscribe() on it.

Analyzing the auth.service.ts Code

Let’s break down your auth.service.ts logic:

Current isLoggedIn Getter

Here’s your original isLoggedIn getter:

[[See Video to Reveal this Text or Code Snippet]]

Problems Identified

Improper Use of Subscriptions: You're subscribing to an observable within the getter and trying to return values with return of(true), which doesn’t work as expected.

No Observable Returned: The getter doesn't return an observable, which causes the subscription in your component to fail.

Proposed Solution

To resolve this issue, you need to modify the isLoggedIn getter so that it directly returns an observable. Here’s how you can implement that:

Revised isLoggedIn Getter

Update your isLoggedIn getter as follows:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Changes

Returning Chains of Observables: We ensure the isLoggedIn getter directly returns an observable from the call to getUserBySession().

Using Operators: The pipe method chains map and catchError to manage the asynchronous logic properly:

map(() => true) emits true if a valid user session is found.

catchError(() => of(false)) will handle any errors during the session check by returning false.

Conclusion

By making these adjustments to your code, you'll ensure that the isLoggedIn property correctly provides an observable, making it possible to subscribe to it in your component without encountering the TypeError. It's essential to understand how observables work within Angular, especially in scenarios involving asynchronous data handling like user authentication.

Implement these changes, and you should find that your error is resolved, enabling smooth and efficient session management in your Angular application.

If you have any questions or need further assistance with your Angular projects, feel free to reach out!