Interceptors for request modification in Angular - Performance & Optimization

Performance: Interceptors for request modification

MEDIUM IMPACT

This affects the time it takes for HTTP requests to be sent and responses to be processed, impacting interaction responsiveness and network efficiency.

Modifying HTTP requests to add headers or tokens

Angular

intercept(req, next) {
  const clonedReq = req.clone({ headers: req.headers.set('Authorization', 'Bearer ' + this.token) });
  return next.handle(clonedReq);
}

Cloning and modifying the request without delays keeps the pipeline fast and responsive.

📈 Performance GainNo added delay, minimal CPU overhead, improves INP by reducing request blocking.

Modifying HTTP requests to add headers or tokens

Angular

intercept(req, next) {
  const clonedReq = req.clone({ headers: req.headers.set('Authorization', 'Bearer ' + this.token) });
  return next.handle(clonedReq).pipe(delay(100));
}

Adding artificial delays or heavy synchronous processing blocks the request pipeline, increasing response time.

📉 Performance CostBlocks request processing for 100ms per request, increasing INP and slowing user interactions.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Interceptor with delay or heavy sync work	0	0	Blocks UI update indirectly by delaying response	[X] Bad
Lightweight interceptor modifying headers only	0	0	Minimal impact on UI update speed	[OK] Good

Rendering Pipeline

Interceptors modify HTTP requests before they leave the browser and responses before they reach components, affecting the network request lifecycle and UI update timing.

→Request Preparation

→Network

→Response Processing

→UI Update

⚠️ BottleneckRequest Preparation and Response Processing when interceptors add heavy synchronous work or delays.

Core Web Vital Affected

INP

This affects the time it takes for HTTP requests to be sent and responses to be processed, impacting interaction responsiveness and network efficiency.

Optimization Tips

1Avoid heavy synchronous work inside interceptors to prevent blocking HTTP requests.

2Do not add artificial delays in interceptors as they increase interaction latency.

3Keep interceptor logic simple and asynchronous to maintain fast response handling.

Performance Quiz - 3 Questions

What is the main performance risk when adding heavy synchronous logic inside an Angular HTTP interceptor?

A It increases the bundle size significantly.

B It blocks the request pipeline, increasing response time and reducing interaction responsiveness.

C It causes layout shifts on the page.

D It improves caching automatically.

DevTools: Performance

How to check: Record a performance profile while triggering HTTP requests. Look for long tasks or delays in the network request timeline caused by interceptor code.

What to look for: Check for blocking time before network requests complete and UI thread idle time to confirm interceptor efficiency.

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