Interceptors for request modification in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Angular HTTP interceptor do to outgoing requests?

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

Angular

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

@Injectable()
export class AuthInterceptor implements HttpInterceptor {
  intercept(req: HttpRequest<any>, next: HttpHandler) {
    const clonedReq = req.clone({
      setHeaders: { Authorization: 'Bearer token123' }
    });
    return next.handle(clonedReq);
  }
}

A Blocks all outgoing HTTP requests and returns an error.

B Removes any Authorization header from outgoing HTTP requests.

C Modifies the request URL to add a query parameter 'token=token123'.

D Adds an Authorization header with value 'Bearer token123' to every outgoing HTTP request.

💡 Hint

Look at the clone method and what headers it sets.

The interceptor clones the original request and adds an Authorization header with the value 'Bearer token123'. This modified request is then passed on.

📝 Syntax

intermediate

Which option correctly modifies the request URL in an Angular interceptor?

You want to add a query parameter 'lang=en' to every outgoing HTTP request URL. Which interceptor code snippet correctly does this?

A const clonedReq = req.clone({ params: req.params.append('lang', 'en') });

B const clonedReq = req.clone({ setParams: { lang: 'en' } });

C const clonedReq = req.clone({ url: req.url + '?lang=en' });

D const clonedReq = req.clone({ queryParams: { lang: 'en' } });

💡 Hint

Check the HttpRequest clone method options for adding query parameters.

The correct way to add query parameters is to use the params property and append the new parameter.

🔧 Debug

advanced

Why does this interceptor fail to add the header?

This interceptor code is intended to add a custom header but it does not work. Why?

Angular

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

@Injectable()
export class CustomHeaderInterceptor implements HttpInterceptor {
  intercept(req: HttpRequest<any>, next: HttpHandler) {
    req.headers.set('X-Custom-Header', 'value123');
    return next.handle(req);
  }
}

A The interceptor forgot to call next.handle(req).

B HttpRequest headers are immutable; set() returns a new headers object but does not modify the original.

C The header name 'X-Custom-Header' is invalid and causes an error.

D The interceptor must use req.clone() to modify the body, not headers.

💡 Hint

Think about immutability of HttpRequest and its headers.

HttpRequest and its headers are immutable. Calling set() on headers returns a new headers object but does not change the original. You must clone the request with the new headers.

❓ state_output

advanced

What is the output of this interceptor chain modifying headers?

Given two interceptors chained in this order, what is the final Authorization header value sent in the HTTP request?

Angular

/* Interceptor 1 */
intercept(req, next) {
  const cloned = req.clone({ setHeaders: { Authorization: 'Token1' } });
  return next.handle(cloned);
}

/* Interceptor 2 */
intercept(req, next) {
  const cloned = req.clone({ setHeaders: { Authorization: 'Token2' } });
  return next.handle(cloned);
}

A 'Token2', because the second interceptor overwrites the header set by the first.

B 'Token1', because the first interceptor sets it and the second does not change it.

C 'Token1, Token2', both headers are combined.

D No Authorization header is sent because of conflict.

💡 Hint

Later interceptors can overwrite headers set by earlier ones.

Each interceptor clones the request and sets the Authorization header. The last interceptor's value overwrites previous ones.

🧠 Conceptual

expert

Which statement about Angular HTTP interceptors is true?

Choose the correct statement about Angular HTTP interceptors and request modification.

A Interceptors can only modify the response, not the request.

B Interceptors can directly modify the HttpRequest object without cloning it.

C Interceptors can modify requests but must always clone the HttpRequest object because it is immutable.

D Interceptors are only called once per application lifetime.

💡 Hint

Think about how Angular handles HttpRequest objects internally.

HttpRequest objects are immutable. Interceptors must clone them to modify headers, URL, or body.