Guard interface (canActivate) in NestJS - Step-by-Step Execution

Concept Flow - Guard interface (canActivate)

Request comes in

↓

Guard's canActivate called

↓

Check condition inside canActivate

↓

Allow

↓

Route

When a request arrives, NestJS calls the guard's canActivate method. If it returns true, the request proceeds. If false, access is blocked.

Execution Sample

NestJS

import { CanActivate, ExecutionContext } from '@nestjs/common';

export class AuthGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean {
    const request = context.switchToHttp().getRequest();
    return !!request.user;
  }
}

This guard checks if the request has a user object. If yes, it allows access; otherwise, it blocks.

Execution Table

Step	Action	Evaluation	Result
1	Request arrives	N/A	Trigger canActivate
2	Extract request from context	request object obtained	request.user exists?
3	Check if request.user is truthy	request.user = undefined	false
4	Return false from canActivate	Access denied	Route handler NOT called
5	Request arrives	N/A	Trigger canActivate
6	Extract request from context	request object obtained	request.user exists?
7	Check if request.user is truthy	request.user = {id:1}	true
8	Return true from canActivate	Access granted	Route handler called

💡 Execution stops after canActivate returns true or false, deciding access.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 7	Final
request	undefined	{ user: undefined }	{ user: undefined }	{ user: { id: 1 } }	{ user: { id: 1 } }
canActivate result	undefined	undefined	false	undefined	true

Key Moments - 3 Insights

Why does canActivate sometimes return false even if the request exists?

Because canActivate checks a condition inside the request, like if request.user exists. The request object always exists, but the user property might be missing, causing false (see execution_table rows 2-4).

What happens if canActivate returns false?

The route handler is NOT called and access is blocked. This is shown in execution_table row 4 where the result is 'Route handler NOT called'.

Can canActivate return a Promise or Observable?

Yes, but in this example it returns a boolean directly. NestJS supports async guards that return Promise<boolean> or Observable<boolean>.

Visual Quiz - 3 Questions

Look at the execution table, what is the canActivate result at step 4?

A true

B false

C undefined

D throws error

💡 Hint

Check the 'Result' column at step 4 in the execution_table.

At which step does the guard allow the route handler to be called?

A Step 4

B Step 3

C Step 8

D Step 2

💡 Hint

Look for 'Route handler called' in the 'Result' column of execution_table.

If request.user is missing, what will canActivate return?

A false

B throws error

C true

D undefined

💡 Hint

See variable_tracker and execution_table rows where request.user is undefined.

Concept Snapshot

Guard interface (canActivate):
- Guards check if a request can proceed.
- canActivate(context) returns true (allow) or false (block).
- Access granted if true, else blocked.
- Can be synchronous or async.
- Used to protect routes in NestJS.

Full Transcript

In NestJS, a Guard is a special class that decides if a request can continue to a route. The key method is canActivate, which receives the request context. Inside canActivate, you check conditions like if the user is logged in. If the check passes, return true to allow access. If not, return false to block. The execution flow starts when a request arrives, NestJS calls canActivate, and based on its boolean result, the route handler runs or not. This example shows canActivate checking if request.user exists. If yes, access is allowed; if no, access is denied. Guards can also return promises or observables for async checks. Understanding this flow helps protect routes easily.

Practice

1. What is the main purpose of the canActivate method in a NestJS guard?

easy

A. To format the response data

B. To handle database connections

C. To decide if a request should be allowed or denied access

D. To log user activity

Solution

1. Step 1: Understand the role of guards in NestJS

   Guards are used to control access to routes by allowing or denying requests.

2. Step 2: Identify the function of canActivate

   The canActivate method returns true or false to allow or deny access.

3. Final Answer:

   To decide if a request should be allowed or denied access -> Option C

4. Quick Check:

   Guard access control = canActivate returns true/false [OK]

Hint: Remember: canActivate returns true to allow access [OK]

Common Mistakes:

• Confusing canActivate with response formatting
• Thinking canActivate handles database logic
• Assuming canActivate logs user actions

2. Which of the following is the correct syntax to implement a guard class with canActivate in NestJS?

easy

A. class MyGuard { canActivate(context) { return true; } }

B. function MyGuard() { canActivate() { return true; } }

C. const MyGuard = () => { canActivate() { return true; } }

D. class MyGuard implements CanActivate { canActivate(context: ExecutionContext): boolean { return true; } }

Solution

1. Step 1: Recognize the NestJS guard pattern

   Guards are classes that implement the CanActivate interface.

2. Step 2: Check the method signature and class syntax

   The method must be named canActivate and accept an ExecutionContext parameter, returning a boolean.

3. Final Answer:

   class MyGuard implements CanActivate { canActivate(context: ExecutionContext): boolean { return true; } } -> Option D

4. Quick Check:

   Guard class implements CanActivate with canActivate method [OK]

Hint: Use class with implements CanActivate and proper method signature [OK]

Common Mistakes:

• Using function instead of class
• Missing implements CanActivate
• Incorrect method parameters or return type

3. Given this guard code, what will be the result when a request is made?

class AlwaysDenyGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean {
    return false;
  }
}

medium

A. The request is denied access

B. The request is allowed to proceed

C. A runtime error occurs

D. The guard is ignored and request proceeds

Solution

1. Step 1: Analyze the canActivate return value

   The method always returns false, meaning access is denied.

2. Step 2: Understand guard effect on request

   When canActivate returns false, NestJS blocks the request from continuing.

3. Final Answer:

   The request is denied access -> Option A

4. Quick Check:

   canActivate false = deny request [OK]

Hint: False return in canActivate means deny access [OK]

Common Mistakes:

• Assuming false allows access
• Thinking guard causes error on false
• Believing guard is skipped if false

4. Identify the error in this guard implementation:

class MyGuard implements CanActivate {
  canActivate() {
    return true;
  }
}

medium

A. Incorrect return type for canActivate

B. Missing parameter in canActivate method

C. Class does not implement CanActivate interface

D. canActivate method should be async

Solution

1. Step 1: Check method signature requirements

   The canActivate method must accept a parameter of type ExecutionContext.

2. Step 2: Compare with given code

   The method has no parameters, which breaks the interface contract.

3. Final Answer:

   Missing parameter in canActivate method -> Option B

4. Quick Check:

   canActivate needs ExecutionContext parameter [OK]

Hint: canActivate always needs ExecutionContext parameter [OK]

Common Mistakes:

• Omitting method parameters
• Assuming async is required
• Ignoring interface implementation rules

5. You want to create a guard that only allows access if the user role is 'admin'. Which of these canActivate implementations correctly checks the role from the request object?

hard

A. canActivate(context: ExecutionContext): boolean {
const request = context.switchToHttp().getRequest();
return request.user?.role === 'admin';
}

B. canActivate(context: ExecutionContext): boolean {
const user = context.getUser();
return user.role === 'admin';
}

C. canActivate(context: ExecutionContext): boolean {
const request = context.getRequest();
return request.user.role === 'admin';
}

D. canActivate(context: ExecutionContext): boolean {
const req = context.switchToHttp().getRequest();
return req.user.role == 'admin';
}

Solution

1. Step 1: Access the HTTP request correctly

   Use context.switchToHttp().getRequest() to get the request object.

2. Step 2: Safely check the user role

   Use optional chaining request.user?.role to avoid errors if user is undefined.

3. Final Answer:

   canActivate(context: ExecutionContext): boolean { const request = context.switchToHttp().getRequest(); return request.user?.role === 'admin'; } -> Option A

4. Quick Check:

   Use switchToHttp and optional chaining for safe role check [OK]

Hint: Use switchToHttp().getRequest() and optional chaining for safety [OK]

Common Mistakes:

• Using context.getUser() which does not exist
• Calling context.getRequest() directly without switchToHttp()
• Not using optional chaining causing runtime errors