Mastering HTTP Interceptors in Angular: Seamless Token Management

When developing modern web applications, securing routes and handling authentication using JWT tokens is a common practice. But what happens when your access token expires? Without a proper token management system, your users could face disrupted sessions and unnecessary logouts.

In this article, I'll walk you through how to implement an Angular HTTP Interceptor to handle JWT token expiration seamlessly by refreshing the token and retrying the failed request — all without interrupting the user experience.

What Are HTTP Interceptors?

In Angular, HTTP interceptors are powerful tools that allow you to intercept and modify outgoing requests or incoming responses before they reach your application. They're ideal for cross-cutting concerns like authentication, logging, and error handling.

For token-based authentication, interceptors are perfect for:

1.     Attaching access tokens to outgoing requests.
2.     Handling expired tokens by refreshing them in the background.
3.     Retrying failed requests after refreshing tokens.

The Problem: Token Expiry

Most web applications use JWTs (JSON Web Tokens) to authorize API requests. However, these tokens typically have a short lifespan for security reasons, which leads to the problem of token expiration. When the access token expires, the user needs to stay authenticated without being logged out abruptly.

The Solution: Refresh Tokens and HTTP Interceptors

We can solve this problem by using refresh tokens. When the access token expires, the refresh token (which is typically long-lived) can be used to obtain a new access token.

The Flow:

1.     Initial Token Retrieval: Upon successful login, we receive an access token and a refresh token. These tokens are stored securely in the browser (e.g., in localStorage).
2.     Adding Tokens to Requests: Every outgoing HTTP request requires the access token in the Authorization header.
3.     Handling Token Expiration: When the server responds with a 401 Unauthorized error (indicating the token has expired), the interceptor triggers a token refresh call using the refresh token. The new access token is then used to retry the original request.
4.     Refreshing Tokens: If the refresh token is valid, a new access token is generated, and the request that originally failed (due to the expired token) is automatically retried.
5.     User Experience: The entire process is transparent to the user. Their session remains active without redirection to the login page.

Angular Implementation

Now, let's dive into how we can implement this in Angular using HTTP Interceptors.
Step 1: User Service

The UserService is responsible for handling token-related operations such as logging in, refreshing tokens, and logging out. 

In this service:

•     getLoginAuthorization handles the initial login and stores both the access and refresh tokens in localStorage.
•     refreshToken fetches a new access token using the refresh token.
•     logout can be used to clear the tokens and log the user out if necessary.

Step 2: Auth Token Interceptor

Next, let’s implement the HTTP interceptor to handle requests, attach tokens, refresh them when they expire, and retry the original request.

Here’s what this interceptor does:

1.     Intercept: Each outgoing HTTP request is intercepted, and the access token is added to the Authorization header.
2.     Error Handling: If the request fails with a 401 Unauthorized error, the interceptor calls handleTokenExpired.
3.     Refresh Token Flow: In handleTokenExpired, a request is made to refresh the token using the refresh token. Once a new access token is received, the original request is retried with the new token.

Step 3: Register the Interceptor

Lastly, register the interceptor in your AppModule:

Key Takeaways

•     Improved User Experience: This approach ensures users stay logged in without interruptions, even when their access token expires.
•     Separation of Concerns: By managing tokens inside the interceptor, we keep the rest of the app clean and focused on business logic.
•     Enhanced Security: Refresh tokens provide a more secure way to manage session persistence than long-lived access tokens.

Conclusion

Handling token-based authentication in Angular apps can seem challenging, but by leveraging the power of HTTP interceptors, we can easily manage token expiration and refresh cycles in the background. This provides a smoother user experience and ensures your app is secure and maintainable.

Have you implemented a similar solution in your projects? I'd love to hear how you've handled token-based authentication in your apps! Let’s connect and discuss.

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