Interceptors for authentication tokens in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Angular HTTP interceptor do?

Consider this Angular HTTP interceptor code. What is the main effect on outgoing HTTP requests?

Angular

import { Injectable } from '@angular/core';
import { HttpInterceptor, HttpRequest, HttpHandler } from '@angular/common/http';

@Injectable({ providedIn: 'root' })
export class AuthInterceptor implements HttpInterceptor {
  intercept(req: HttpRequest<any>, next: HttpHandler) {
    const token = localStorage.getItem('authToken');
    if (token) {
      const cloned = req.clone({
        headers: req.headers.set('Authorization', `Bearer ${token}`)
      });
      return next.handle(cloned);
    }
    return next.handle(req);
  }
}

A Intercepts HTTP responses to check for token expiration and logs out the user.

B Removes any Authorization header from outgoing HTTP requests to prevent token leaks.

C Adds an Authorization header with a Bearer token to all outgoing HTTP requests if a token exists in localStorage.

D Modifies the request URL to include the token as a query parameter for authentication.

💡 Hint

Look at how the request is cloned and what header is added before sending.

This interceptor checks if a token exists in localStorage. If yes, it clones the request and adds an Authorization header with the Bearer token. This header is then sent with the request.

📝 Syntax

intermediate

Identify the syntax error in this Angular interceptor snippet

Which option correctly fixes the syntax error in this Angular HTTP interceptor code?

Angular

intercept(req: HttpRequest<any>, next: HttpHandler) {
  const token = localStorage.getItem('authToken');
  if(token) {
    const cloned = req.clone({
      headers: req.headers.set('Authorization', 'Bearer ' + token)
    });
    return next.handle(cloned);
  }
  return next.handle(req);
}

A Add a missing semicolon after localStorage.getItem('authToken')

B Change req.clone to req.clone() with empty parentheses

C Add a missing colon after the intercept method name

D Add a missing comma after 'Authorization' in headers.set

💡 Hint

Check the line where token is assigned from localStorage.

JavaScript and TypeScript allow omitting semicolons but Angular style guide recommends them. Here, missing semicolon after localStorage.getItem('authToken') can cause issues in some setups.

🔧 Debug

advanced

Why does this interceptor fail to add the token header?

This Angular interceptor is supposed to add an Authorization header but the header is missing in requests. What is the cause?

Angular

intercept(req: HttpRequest<any>, next: HttpHandler) {
  const token = localStorage.getItem('authToken');
  if (token) {
    req.headers.set('Authorization', `Bearer ${token}`);
  }
  return next.handle(req);
}

A HttpRequest objects are immutable; headers.set returns a new headers object but does not modify the original request.

B localStorage.getItem returns null if no token, causing a runtime error.

C The interceptor must call next.handle() inside the if block only.

D The Authorization header must be lowercase 'authorization' to be recognized.

💡 Hint

Think about how Angular HttpRequest objects handle immutability.

HttpRequest objects are immutable. Calling headers.set returns a new headers object but does not change the original request. The interceptor must clone the request with the new headers.

❓ state_output

advanced

What is the output of this interceptor chain?

Given two interceptors registered in this order: AuthInterceptor adds Authorization header if token exists; LoggerInterceptor logs request headers. What will LoggerInterceptor log if localStorage has token 'abc123'?

Angular

AuthInterceptor intercept(req, next) {
  const token = localStorage.getItem('authToken');
  if (token) {
    const cloned = req.clone({ headers: req.headers.set('Authorization', `Bearer ${token}`) });
    return next.handle(cloned);
  }
  return next.handle(req);
}

LoggerInterceptor intercept(req, next) {
  console.log(req.headers.get('Authorization'));
  return next.handle(req);
}

A LoggerInterceptor logs null because headers are not set yet

B LoggerInterceptor logs 'Bearer abc123'

C LoggerInterceptor logs undefined due to missing header

D LoggerInterceptor logs an error because req.headers.get is not a function

💡 Hint

Remember interceptors run in the order they are registered.

Interceptors run in registration order. AuthInterceptor adds the header before passing the cloned request to LoggerInterceptor, so LoggerInterceptor sees the header.

🧠 Conceptual

expert

Why use an HTTP interceptor for authentication tokens instead of adding headers manually in each service?

Which is the best reason to use an Angular HTTP interceptor to add authentication tokens rather than setting headers manually in every HTTP call?

A Interceptors allow tokens to be encrypted automatically without extra code.

B Interceptors improve HTTP request speed by caching tokens locally in the browser.

C Interceptors prevent the need for Angular services to import HttpClient.

D Interceptors centralize token handling, reduce code duplication, and automatically add tokens to all requests, improving maintainability and security.

💡 Hint

Think about code reuse and consistency across many HTTP calls.

Using interceptors centralizes the logic to add tokens, so you don't repeat code in every service. This makes your app easier to maintain and ensures all requests have the token.