Route guards (canActivate, canDeactivate) in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when canActivate guard returns false?

Consider an Angular route with a canActivate guard that returns false. What is the behavior when a user tries to navigate to this route?

A The navigation proceeds and the route component loads normally.

B The navigation is cancelled and the user stays on the current page.

C The application throws a runtime error and crashes.

D The router redirects automatically to the root path.

💡 Hint

Think about what a guard returning false means for navigation.

If a canActivate guard returns false, Angular cancels the navigation. The user remains on the current page and the new route does not load.

❓ state_output

intermediate

What is the value of a flag after canDeactivate guard returns true?

In an Angular component, a canDeactivate guard checks a flag isSaved. If the guard returns true, what can we say about the isSaved flag after navigation away?

Angular

class EditComponent {
  isSaved = false;
  // canDeactivate guard checks this flag
}

A isSaved is false, meaning changes are not saved but navigation allowed.

B isSaved is true, meaning changes are saved before leaving.

C isSaved is undefined because the guard does not affect it.

D isSaved is reset to null automatically by Angular.

💡 Hint

The guard returning true means navigation is allowed, but does it guarantee saving?

The canDeactivate guard returning true means navigation is allowed. It does not guarantee that isSaved is true. The flag remains as it was.

📝 Syntax

advanced

Which option correctly implements a canActivate guard returning an Observable?

Select the correct Angular canActivate guard method that returns an Observable allowing navigation only if a user is logged in.

Angular

import { Injectable } from '@angular/core';
import { CanActivate } from '@angular/router';
import { Observable, of } from 'rxjs';

@Injectable({ providedIn: 'root' })
export class AuthGuard implements CanActivate {
  isLoggedIn = false;

  canActivate(): Observable<boolean> {
    // fill in correct return
  }
}

A return new Promise(resolve => resolve(this.isLoggedIn));

B return this.isLoggedIn ? true : false;

C return of(this.isLoggedIn);

D return Observable.of(this.isLoggedIn);

💡 Hint

Use RxJS to return an Observable of a boolean value.

The canActivate method must return an Observable. Using of(this.isLoggedIn) creates an Observable emitting the boolean value.

Option C returns a boolean, not an Observable. Option C returns a Promise, not Observable. Option C uses deprecated syntax.

🔧 Debug

advanced

Why does this canDeactivate guard cause a runtime error?

Given this canDeactivate guard code, why does it cause a runtime error when navigating away?

canDeactivate(component: any): boolean {
  return component.isSaved && component.hasPermission();
}

Angular

class EditComponent {
  isSaved = true;
  // hasPermission method is missing
}

A The guard returns false always, blocking navigation.

B component.isSaved is undefined, causing a ReferenceError.

C The guard returns a Promise instead of boolean, causing error.

D component.hasPermission is not a function, causing a TypeError.

💡 Hint

Check if all methods used on component exist.

The guard calls component.hasPermission(), but the component does not have this method. This causes a TypeError at runtime.

🧠 Conceptual

expert

What is the effect of returning a UrlTree from canActivate guard?

In Angular routing, if a canActivate guard returns a UrlTree instead of true or false, what happens?

A The router cancels current navigation and redirects to the UrlTree path.

B The router ignores the UrlTree and proceeds with navigation.

C The router throws a syntax error because UrlTree is invalid return type.

D The router navigates to both the original and UrlTree paths simultaneously.

💡 Hint

Think about how Angular handles redirect instructions from guards.

Returning a UrlTree from a canActivate guard tells Angular to cancel the current navigation and redirect to the URL represented by the UrlTree.

Practice

1. What is the primary purpose of the canActivate guard in Angular routing?

easy

A. To handle errors during route navigation.

B. To check if navigation to a route is allowed before entering it.

C. To preload data before the route loads.

D. To confirm if navigation away from a route is allowed.

Solution

1. Step 1: Understand the role of canActivate

   canActivate is a route guard that runs before entering a route to decide if navigation is allowed.

2. Step 2: Differentiate from other guards

   canDeactivate checks leaving a route, not entering. Preloading and error handling are unrelated to canActivate.

3. Final Answer:

   To check if navigation to a route is allowed before entering it. -> Option B

4. Quick Check:

   canActivate = check before entering route [OK]

Hint: canActivate controls entering routes, canDeactivate controls leaving [OK]

Common Mistakes:

• Confusing canActivate with canDeactivate
• Thinking canActivate preloads data
• Assuming canActivate handles errors

2. Which of the following is the correct syntax to apply a canDeactivate guard to a route in Angular?

easy

A. { path: 'edit', component: EditComponent, canDeactivate: [UnsavedChangesGuard] }

B. { path: 'edit', component: EditComponent, canLoad: [LoadGuard] }

C. { path: 'edit', component: EditComponent, canActivate: [AuthGuard] }

D. { path: 'edit', component: EditComponent, canActivateChild: [ChildGuard] }

Solution

1. Step 1: Identify the guard type for leaving routes

   canDeactivate is used to check if navigation away from a route is allowed.

2. Step 2: Match syntax with canDeactivate

   The correct syntax uses canDeactivate: [GuardName] inside the route object.

3. Final Answer:

   { path: 'edit', component: EditComponent, canDeactivate: [UnsavedChangesGuard] } -> Option A

4. Quick Check:

   canDeactivate syntax = canDeactivate: [Guard] [OK]

Hint: Use canDeactivate: [Guard] to protect leaving a route [OK]

Common Mistakes:

• Using canActivate instead of canDeactivate for leaving routes
• Confusing canLoad with canDeactivate
• Missing brackets around guard name

3. Given this canDeactivate guard code snippet, what will happen when the user tries to leave the EditComponent with unsaved changes?

export class UnsavedChangesGuard implements CanDeactivate<EditComponent> {
  canDeactivate(component: EditComponent): boolean {
    if (component.hasUnsavedChanges()) {
      return confirm('You have unsaved changes. Leave anyway?');
    }
    return true;
  }
}

medium

A. The guard throws an error because confirm is not allowed.

B. The user is always allowed to leave without confirmation.

C. The user is blocked from leaving if there are unsaved changes.

D. The user is asked to confirm leaving only if there are unsaved changes.

Solution

1. Step 1: Analyze the guard logic

   The guard checks if component.hasUnsavedChanges() returns true, then asks for confirmation.

2. Step 2: Understand the confirm behavior

   If the user confirms, confirm() returns true allowing navigation; otherwise, navigation is blocked.

3. Final Answer:

   The user is asked to confirm leaving only if there are unsaved changes. -> Option D

4. Quick Check:

   canDeactivate confirms only if unsaved changes exist [OK]

Hint: canDeactivate can prompt user to confirm leaving [OK]

Common Mistakes:

• Assuming user can leave without confirmation always
• Thinking guard blocks leaving without prompt
• Believing confirm causes errors in Angular guards

4. Identify the error in this canActivate guard implementation:

export class AuthGuard implements CanActivate {
  canActivate(route: ActivatedRouteSnapshot, state: RouterStateSnapshot) {
    if (this.authService.isLoggedIn()) {
      return true;
    }
  }
}

medium

A. Missing return statement when user is not logged in.

B. Incorrect method name; should be canDeactivate.

C. The guard should return a Promise, not boolean.

D. The guard must not use services inside canActivate.

Solution

1. Step 1: Check all code paths return a value

   If isLoggedIn() is false, the method returns nothing (undefined), which is invalid.

2. Step 2: Understand guard return requirements

   canActivate must return true, false, or an observable/promise resolving to those. Missing return causes errors.

3. Final Answer:

   Missing return statement when user is not logged in. -> Option A

4. Quick Check:

   All canActivate paths must return true/false [OK]

Hint: Always return true or false in all canActivate paths [OK]

Common Mistakes:

• Not returning false or redirect when unauthorized
• Confusing canActivate with canDeactivate method names
• Thinking services can't be used in guards

5. You want to prevent users from leaving a form page if they have unsaved changes, but only if they are editing (not creating new). Which approach correctly uses canDeactivate to achieve this?

hard

A. Use canActivate guard to check unsaved changes before entering the form route.

B. Use canDeactivate guard that always returns true to allow navigation.

C. Use canDeactivate guard that checks component.isEditing and component.hasUnsavedChanges() before confirming navigation.

D. Use canActivateChild guard to block leaving the form.

Solution

1. Step 1: Identify when to block navigation away

   Blocking leaving only applies when editing and unsaved changes exist, so guard must check both conditions.

2. Step 2: Use canDeactivate guard properly

   canDeactivate runs when leaving a route, so it is the correct place to check and confirm navigation.

3. Final Answer:

   Use canDeactivate guard that checks component.isEditing and component.hasUnsavedChanges() before confirming navigation. -> Option C

4. Quick Check:

   canDeactivate with condition on editing and unsaved changes [OK]

Hint: Check editing and unsaved changes inside canDeactivate guard [OK]

Common Mistakes:

• Using canActivate to block leaving routes
• Always returning true in canDeactivate guard
• Confusing canActivateChild with canDeactivate