Interceptors for authentication tokens in Angular - Performance & Optimization

Performance: Interceptors for authentication tokens

MEDIUM IMPACT

This affects the network request speed and page responsiveness by modifying HTTP requests and responses before they reach the server or the app.

Adding authentication tokens to HTTP requests

Angular

intercept(req, next) {
  return from(this.authService.getTokenAsync()).pipe(
    switchMap(token => {
      const authReq = req.clone({ headers: req.headers.set('Authorization', `Bearer ${token}`) });
      return next.handle(authReq);
    })
  );
}

Asynchronous token retrieval avoids blocking the main thread and allows requests to proceed smoothly.

📈 Performance GainNon-blocking requests improve input responsiveness (INP) and reduce UI delays

Adding authentication tokens to HTTP requests

Angular

intercept(req, next) {
  const token = this.authService.getTokenSync();
  const authReq = req.clone({ headers: req.headers.set('Authorization', `Bearer ${token}`) });
  return next.handle(authReq);
}

Synchronous token retrieval blocks the interceptor, delaying all HTTP requests and hurting responsiveness.

📉 Performance CostBlocks rendering for 50-100ms per request depending on token retrieval speed

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Synchronous token retrieval in interceptor	0	0	Blocks UI thread causing delayed input response	[X] Bad
Asynchronous token retrieval with RxJS in interceptor	0	0	Non-blocking, smooth UI interaction	[OK] Good

Rendering Pipeline

Interceptors modify HTTP requests before they are sent and responses before they are processed, affecting network timing and UI update speed.

→Network Request

→JavaScript Execution

→UI Update

⚠️ BottleneckJavaScript Execution when synchronous blocking occurs

Core Web Vital Affected

INP

This affects the network request speed and page responsiveness by modifying HTTP requests and responses before they reach the server or the app.

Optimization Tips

1Avoid synchronous token retrieval in interceptors to prevent blocking HTTP requests.

2Use asynchronous methods like RxJS to fetch tokens without blocking the UI thread.

3Keep interceptor logic minimal to reduce JavaScript execution time and improve responsiveness.

Performance Quiz - 3 Questions

What is the main performance risk of retrieving authentication tokens synchronously in an Angular interceptor?

A It blocks the main thread delaying all HTTP requests and UI updates.

B It increases the bundle size significantly.

C It causes layout shifts on the page.

D It reduces the number of HTTP requests.

DevTools: Performance

How to check: Record a performance profile while triggering HTTP requests. Look for long tasks or blocking scripts during request interception.

What to look for: Check for long JavaScript execution times in the flame chart indicating blocking synchronous token retrieval.

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