Interceptors for authentication tokens in Angular - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an HTTP interceptor in Angular?

An HTTP interceptor is a service that can inspect and transform HTTP requests or responses globally before they reach the server or the app. It helps add headers, handle errors, or modify requests centrally.

beginner

Why use an interceptor for authentication tokens?

Using an interceptor lets you automatically add authentication tokens to every HTTP request without repeating code in each service. This keeps your code clean and secure.

intermediate

How do you provide an interceptor in Angular?

You provide an interceptor by adding it to the providers array with the token HTTP_INTERCEPTORS and setting multi: true. This tells Angular to use your interceptor alongside others.

intermediate

What method must an Angular interceptor implement?

An Angular interceptor must implement the intercept(req, next) method. It receives the outgoing request and the next handler, and returns an Observable of the HTTP event.

advanced

How can you handle token expiration in an interceptor?

You can check the server response for 401 errors in the interceptor, then trigger a token refresh or redirect to login. This centralizes token expiration handling.

What does an Angular HTTP interceptor primarily do?

A Create new HTTP requests manually

B Handle UI events

C Replace Angular services

D Modify HTTP requests and responses globally

Interceptors modify HTTP requests and responses before they reach the server or the app.

How do you add an interceptor to Angular's HTTP pipeline?

A Import it in the component directly

B Add it to the providers array with HTTP_INTERCEPTORS and multi: true

C Call it inside each service method

D Declare it in the module's declarations

Interceptors must be provided with HTTP_INTERCEPTORS token and multi: true in providers.

Which method must be implemented in an Angular HTTP interceptor?

A intercept(req, next)

B handleRequest()

C sendRequest()

D processHttp()

The intercept method handles the request and forwards it.

What is a common use of interceptors in authentication?

A Add authentication tokens to HTTP headers

B Encrypt the entire HTTP request body

C Create new user accounts

D Render login forms

Interceptors add tokens to headers to authenticate requests.

How can an interceptor respond to a 401 Unauthorized error?

A Reload the entire app

B Ignore the error silently

C Trigger token refresh or redirect to login

D Automatically approve the request

Interceptors can catch 401 errors to refresh tokens or redirect users.

Explain how an Angular HTTP interceptor can be used to add authentication tokens to requests.

Think about modifying the request before sending it.

Describe how you would handle expired tokens using an Angular interceptor.

Focus on response handling inside the interceptor.

Practice

1. What is the main purpose of an HTTP interceptor in Angular when dealing with authentication tokens?

easy

A. To create new HTTP requests manually

B. To automatically add authentication tokens to outgoing HTTP requests

C. To block all HTTP requests from the app

D. To change the URL of HTTP requests

Solution

1. Step 1: Understand the role of interceptors

   Interceptors are used to modify HTTP requests or responses globally in Angular.

2. Step 2: Identify the use of tokens in interceptors

   They commonly add authentication tokens to requests to keep the app secure without repeating code.

3. Final Answer:

   To automatically add authentication tokens to outgoing HTTP requests -> Option B

4. Quick Check:

   Interceptors add tokens = B [OK]

Hint: Interceptors modify requests automatically, especially for tokens [OK]

Common Mistakes:

• Thinking interceptors create requests
• Believing interceptors block requests
• Assuming interceptors change URLs

2. Which of the following is the correct way to clone an HTTP request before adding an authentication token in an Angular interceptor?

easy

A. request.updateHeaders({ Authorization: 'Bearer ' + token })

B. request.headers.set('Authorization', 'Bearer ' + token)

C. request.modifyHeaders('Authorization', 'Bearer ' + token)

D. request.clone({ headers: request.headers.set('Authorization', 'Bearer ' + token) })

Solution

1. Step 1: Recall Angular HttpRequest immutability

   HttpRequest objects are immutable, so you must clone them to modify headers.

2. Step 2: Use the clone method with headers.set()

   The correct syntax is to call clone() and set the new header inside it.

3. Final Answer:

   request.clone({ headers: request.headers.set('Authorization', 'Bearer ' + token) }) -> Option D

4. Quick Check:

   Clone request with headers.set() = A [OK]

Hint: Always clone requests before changing headers [OK]

Common Mistakes:

• Trying to modify headers directly without cloning
• Using non-existent methods like modifyHeaders or updateHeaders
• Not using the clone method

3. Given this Angular interceptor code snippet, what will be the Authorization header value in the outgoing HTTP request if the token is 'abc123'?

intercept(request: HttpRequest<any>, next: HttpHandler) {
  const token = 'abc123';
  const cloned = request.clone({
    headers: request.headers.set('Authorization', `Bearer ${token}`)
  });
  return next.handle(cloned);
}

medium

A. Authorization: Bearer abc123

B. Authorization: abc123

C. Authorization: Bearer token

D. No Authorization header will be added

Solution

1. Step 1: Check how the token is added

   The code sets the Authorization header to 'Bearer ' plus the token string 'abc123'.

2. Step 2: Understand string interpolation

   The template string `Bearer ${token}` becomes 'Bearer abc123'.

3. Final Answer:

   Authorization: Bearer abc123 -> Option A

4. Quick Check:

   Header value = 'Bearer abc123' [OK]

Hint: Template strings insert token value directly [OK]

Common Mistakes:

• Forgetting 'Bearer ' prefix
• Using the literal word 'token' instead of variable
• Assuming header is not added

4. Identify the error in this Angular interceptor code that tries to add an auth token:

intercept(req: HttpRequest<any>, next: HttpHandler) {
  const token = 'xyz';
  req.headers.set('Authorization', 'Bearer ' + token);
  return next.handle(req);
}

medium

A. Headers are immutable; must clone request before setting headers

B. Token variable is not defined

C. next.handle() is missing a return statement

D. Authorization header should be lowercase

Solution

1. Step 1: Understand HttpRequest immutability

   HttpRequest headers cannot be changed directly; they are immutable.

2. Step 2: Correct way is to clone the request with new headers

   You must clone the request and set headers on the clone before passing it on.

3. Final Answer:

   Headers are immutable; must clone request before setting headers -> Option A

4. Quick Check:

   Immutable headers require cloning = A [OK]

Hint: Headers are immutable; clone request before modifying [OK]

Common Mistakes:

• Trying to set headers directly on request
• Ignoring immutability of HttpRequest
• Not returning next.handle()

5. You want to create an Angular interceptor that adds an authentication token only to requests targeting your API domain 'api.example.com'. Which approach correctly implements this conditional logic?

hard

A. Modify the token inside next.handle() without cloning

B. Add the token to all requests unconditionally

C. Check if request.url.includes('api.example.com') before cloning and adding the token

D. Use a global variable to store the token and add it after the request completes

Solution

1. Step 1: Identify the need for conditional token addition

   Only requests to 'api.example.com' should have the token added to avoid leaking tokens to other domains.

2. Step 2: Use URL check before cloning and modifying request

   Check request.url.includes('api.example.com') and then clone the request with the token header.

3. Step 3: Avoid modifying requests without cloning or adding tokens globally

   Modifying without cloning or adding tokens to all requests is insecure or incorrect.

4. Final Answer:

   Check if request.url.includes('api.example.com') before cloning and adding the token -> Option C

5. Quick Check:

   Conditional token addition by URL check = C [OK]

Hint: Add tokens only after checking request URL [OK]

Common Mistakes:

• Adding tokens to all requests blindly
• Modifying requests without cloning
• Trying to add tokens after request completes