Interceptors for request modification 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 before they are handled by the application. It acts like a middleman to modify requests or responses globally.

beginner

How do you register an HTTP interceptor in Angular?

You provide the interceptor class in the providers array with the token HTTP_INTERCEPTORS and set multi: true to allow multiple interceptors.

intermediate

What method must an Angular HTTP interceptor implement?

An Angular HTTP interceptor must implement the intercept(req: HttpRequest<any>, next: HttpHandler) method, which returns an Observable of the HTTP event stream.

intermediate

Why use interceptors to modify HTTP requests instead of modifying requests in each service?

Interceptors allow centralized request modification, reducing repeated code and ensuring consistent behavior like adding headers or logging across all HTTP calls.

beginner

Can Angular HTTP interceptors modify both requests and responses?

Yes, interceptors can modify outgoing requests before they are sent and also inspect or modify incoming responses before they reach the component.

Which Angular token is used to register HTTP interceptors?

A HTTP_CLIENT

B HTTP_INTERCEPTORS

C HTTP_HANDLER

D HTTP_SERVICE

HTTP_INTERCEPTORS is the Angular injection token used to register one or more HTTP interceptors.

What does the intercept method receive as parameters?

A HttpRequest and HttpHandler

B HttpResponse and HttpClient

C HttpEvent and HttpHeaders

D HttpClient and HttpInterceptor

The intercept method receives the HttpRequest to modify and the HttpHandler to forward the request.

How do you ensure multiple interceptors can be used together?

A Set multi: true when providing interceptors

B Use only one interceptor at a time

C Register interceptors in different modules

D Use different tokens for each interceptor

Setting multi: true allows Angular to keep multiple interceptors in an array and apply them all.

Which of these is a common use case for HTTP interceptors?

A Handling user input events

B Rendering UI components

C Adding authentication tokens to headers

D Styling HTML elements

Interceptors often add authentication tokens or custom headers to outgoing HTTP requests.

What type does the intercept method return?

A void

B Promise<HttpResponse>

C HttpRequest

D Observable<HttpEvent<any>>

The intercept method returns an Observable stream of HttpEvent objects.

Explain how to create and register an HTTP interceptor in Angular to add a custom header to all requests.

Think about modifying the request inside intercept and registering the interceptor in providers.

Describe why using HTTP interceptors is better than modifying HTTP requests individually in each service.

Consider the benefits of handling cross-cutting concerns in one place.

Practice

1. What is the main purpose of an HTTP interceptor in Angular?

easy

A. To modify HTTP requests before they are sent

B. To create new HTTP requests from scratch

C. To handle user input events

D. To style components dynamically

Solution

1. Step 1: Understand the role of interceptors

   Interceptors are designed to catch HTTP requests and responses to modify or handle them.

2. Step 2: Identify the correct purpose

   Among the options, only modifying HTTP requests before sending matches the interceptor's role.

3. Final Answer:

   To modify HTTP requests before they are sent -> Option A

4. Quick Check:

   Interceptor purpose = modify requests [OK]

Hint: Interceptors change requests before sending [OK]

Common Mistakes:

• Thinking interceptors create requests
• Confusing interceptors with event handlers
• Assuming interceptors style components

2. Which method is used to safely modify an HTTP request inside an Angular interceptor?

easy

A. req.clone()

B. req.change()

C. req.update()

D. req.modify()

Solution

1. Step 1: Recall Angular HTTP request immutability

   HTTP requests are immutable, so you cannot change them directly.

2. Step 2: Use the clone method to modify

   The correct way is to create a copy with changes using req.clone().

3. Final Answer:

   req.clone() -> Option A

4. Quick Check:

   Modify request = req.clone() [OK]

Hint: Use req.clone() to change requests safely [OK]

Common Mistakes:

• Trying to modify request directly
• Using non-existent methods like req.modify()
• Confusing clone with update or change

3. Given this interceptor code snippet, what will be the value of the 'Authorization' header in the outgoing request?

intercept(req, next) {
  const cloned = req.clone({
    setHeaders: { Authorization: 'Bearer token123' }
  });
  return next.handle(cloned);
}

medium

A. An error will occur due to incorrect syntax

B. No Authorization header will be set

C. 'token123'

D. 'Bearer token123'

Solution

1. Step 1: Analyze the clone method usage

   The code clones the request and sets the 'Authorization' header to 'Bearer token123'.

2. Step 2: Understand header setting effect

   The cloned request will have the header exactly as set, so the header value is 'Bearer token123'.

3. Final Answer:

   'Bearer token123' -> Option D

4. Quick Check:

   Header set to 'Bearer token123' [OK]

Hint: Headers set in clone appear in outgoing request [OK]

Common Mistakes:

• Assuming header is missing
• Confusing header value with token only
• Thinking syntax causes error

4. Identify the error in this Angular interceptor code:

intercept(req, next) {
  req.setHeaders({ 'Content-Type': 'application/json' });
  return next.handle(req);
}

medium

A. Missing return statement

B. req.setHeaders is not a function

C. next.handle should be called with a cloned request

D. Headers cannot be set in interceptors

Solution

1. Step 1: Check method existence on req

   The HttpRequest object is immutable and does not have a setHeaders method.

2. Step 2: Understand correct way to set headers

   Headers must be set by cloning the request with new headers using req.clone().

3. Final Answer:

   req.setHeaders is not a function -> Option B

4. Quick Check:

   setHeaders method does not exist [OK]

Hint: HttpRequest is immutable; no setHeaders method [OK]

Common Mistakes:

• Trying to call setHeaders directly
• Ignoring immutability of HttpRequest
• Thinking headers can't be changed at all

5. You want to add a custom header 'X-App-Version' with value '1.0' to all outgoing HTTP requests in your Angular app. Which is the best way to do this?

hard

A. Add the header manually in every HTTP call

B. Modify the HttpClient service to include the header

C. Create an interceptor that clones requests and sets the header, then register it globally

D. Use a component to add the header before each request

Solution

1. Step 1: Understand global header addition

   Manually adding headers everywhere is error-prone and repetitive.

2. Step 2: Use interceptor for centralized modification

   Interceptors allow modifying all requests in one place by cloning and setting headers, then registering globally.

3. Final Answer:

   Create an interceptor that clones requests and sets the header, then register it globally -> Option C

4. Quick Check:

   Global header addition = interceptor + clone + register [OK]

Hint: Use interceptor to add headers globally [OK]

Common Mistakes:

• Adding headers manually in every call
• Trying to modify HttpClient directly
• Using components for HTTP header changes