Built-in pipes (ParseIntPipe, ParseBoolPipe) in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when ParseIntPipe receives a non-numeric string?

Consider a NestJS controller method using @Param('id', ParseIntPipe). What will happen if the URL parameter id is 'abc'?

NestJS

import { Controller, Get, Param, ParseIntPipe } from '@nestjs/common';

@Controller('items')
export class ItemsController {
  @Get(':id')
  getItem(@Param('id', ParseIntPipe) id: number) {
    return { id };
  }
}

A The method will receive the string 'abc' as the id parameter without error.

B The method will receive NaN as the id parameter.

C The request will fail with a 400 Bad Request error because 'abc' cannot be converted to a number.

D The method will receive the number 0 as the id parameter.

💡 Hint

ParseIntPipe tries to convert the input to a number and throws if it fails.

The ParseIntPipe automatically converts the input to a number. If the input is not a valid number string, it throws a BadRequestException, resulting in a 400 error response.

❓ state_output

intermediate

What value does ParseBoolPipe produce for 'false' string?

Using @Query('active', ParseBoolPipe) in a NestJS controller, what will be the value of active if the query parameter is ?active=false?

NestJS

import { Controller, Get, Query, ParseBoolPipe } from '@nestjs/common';

@Controller('users')
export class UsersController {
  @Get()
  getUsers(@Query('active', ParseBoolPipe) active: boolean) {
    return { active };
  }
}

A The value of active will be true because any string is truthy.

B The value of active will be the string 'false'.

C The request will fail with a 400 Bad Request error.

D The value of active will be the boolean false.

💡 Hint

ParseBoolPipe converts 'true' and 'false' strings to booleans.

The ParseBoolPipe converts the string 'false' to the boolean false. It only accepts 'true' or 'false' (case insensitive) and throws an error otherwise.

📝 Syntax

advanced

Which option correctly applies ParseIntPipe to a route parameter?

Select the correct NestJS controller method signature that uses ParseIntPipe to convert the userId route parameter to a number.

A

@Get(':userId')
getUser(@Param('userId', ParseIntPipe) userId: number) { return userId; }

B

@Get(':userId')
getUser(@Param(ParseIntPipe) userId: number) { return userId; }

C

@Get(':userId')
getUser(@Param('userId') userId: ParseIntPipe) { return userId; }

D

@Get(':userId')
getUser(@Param('userId') userId: number, ParseIntPipe) { return userId; }

💡 Hint

The pipe is passed as the second argument to @Param decorator.

The correct syntax is @Param('paramName', Pipe). Option A correctly applies ParseIntPipe to the userId parameter.

🔧 Debug

advanced

Why does ParseBoolPipe throw an error for query '?flag=yes'?

Given this controller method:

getFlag(@Query('flag', ParseBoolPipe) flag: boolean) { return flag; }

What causes the error when the query string is ?flag=yes?

A ParseBoolPipe converts 'yes' to true, so no error occurs.

B ParseBoolPipe only accepts 'true' or 'false' strings; 'yes' is invalid and causes a 400 error.

C The query parameter 'flag' is missing, causing a null error.

D The controller method is missing a return statement, causing an error.

💡 Hint

ParseBoolPipe expects specific string values.

ParseBoolPipe only accepts the strings 'true' or 'false' (case insensitive). Any other string, like 'yes', causes it to throw a BadRequestException.

🧠 Conceptual

expert

How do ParseIntPipe and ParseBoolPipe improve API robustness?

Choose the best explanation of how using ParseIntPipe and ParseBoolPipe in NestJS controllers helps API reliability.

A They automatically validate and convert input parameters, preventing invalid data from reaching business logic and returning clear errors to clients.

B They cache parsed values to improve performance of repeated requests.

C They encrypt input parameters to secure sensitive data during transmission.

D They log all incoming parameters for auditing purposes.

💡 Hint

Think about input validation and error handling.

These pipes validate and convert inputs before the controller logic runs. This stops invalid data early and sends clear error responses, making APIs more robust and easier to maintain.

Practice

1. What is the main purpose of NestJS built-in pipes like ParseIntPipe and ParseBoolPipe?

easy

A. To create new HTTP routes in the application

B. To format output data before sending it to the client

C. To handle database connections automatically

D. To automatically convert and validate input types like numbers and booleans

Solution

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

   Pipes transform and validate incoming data before it reaches the route handler.

2. Step 2: Identify what ParseIntPipe and ParseBoolPipe do

   They convert string inputs to integer and boolean types respectively, ensuring correct data types.

3. Final Answer:

   To automatically convert and validate input types like numbers and booleans -> Option D

4. Quick Check:

   Pipe purpose = convert and validate input [OK]

Hint: Pipes transform inputs before controller methods run [OK]

Common Mistakes:

• Confusing pipes with output formatting
• Thinking pipes manage database or routing
• Assuming pipes only validate without conversion

2. Which of the following is the correct way to use ParseIntPipe in a NestJS controller method parameter?

easy

A. @Body('id', ParseIntPipe) id: string

B. @Param('id', ParseIntPipe) id: string

C. @Param('id', ParseIntPipe) id: number

D. @Query('id', ParseIntPipe) id: boolean

Solution

1. Step 1: Check the parameter decorator and pipe usage

   Using @Param('id', ParseIntPipe) applies the pipe to convert the 'id' route parameter.

2. Step 2: Match the parameter type with the pipe's output

   ParseIntPipe converts input to a number, so the parameter type must be number.

3. Final Answer:

   @Param('id', ParseIntPipe) id: number -> Option C

4. Quick Check:

   ParseIntPipe converts to number, so type = number [OK]

Hint: ParseIntPipe means parameter type must be number [OK]

Common Mistakes:

• Using string type with ParseIntPipe
• Applying pipe to wrong decorator like @Body for route param
• Assigning boolean type with ParseIntPipe

3. Given this NestJS controller method:

@Get(':active')
getStatus(@Param('active', ParseBoolPipe) active: boolean) {
  return active ? 'Active' : 'Inactive';
}

What will be the response if the client requests /true?

medium

A. "Active"

B. "Inactive"

C. 400 Bad Request error

D. "true"

Solution

1. Step 1: Understand ParseBoolPipe behavior

   ParseBoolPipe converts string 'true' to boolean true.

2. Step 2: Evaluate the method return based on boolean value

   Since active is true, the method returns 'Active'.

3. Final Answer:

   "Active" -> Option A

4. Quick Check:

   Input 'true' -> boolean true -> returns 'Active' [OK]

Hint: ParseBoolPipe converts 'true' string to boolean true [OK]

Common Mistakes:

• Expecting string 'true' instead of boolean
• Assuming 400 error for valid boolean string
• Confusing output with input string

4. Consider this NestJS controller method:

@Get(':id')
getUser(@Param('id', ParseIntPipe) id: number) {
  return `User ID is ${id}`;
}

If a client sends a request to /abc, what will happen?

medium

A. Throws a 400 Bad Request error

B. Returns 'User ID is NaN'

C. Returns 'User ID is abc'

D. Returns 'User ID is 0'

Solution

1. Step 1: Understand ParseIntPipe input validation

   ParseIntPipe tries to convert 'abc' to a number but fails.

2. Step 2: Identify NestJS behavior on invalid pipe input

   When conversion fails, NestJS returns a 400 Bad Request error automatically.

3. Final Answer:

   Throws a 400 Bad Request error -> Option A

4. Quick Check:

   Invalid int input -> 400 error [OK]

Hint: Invalid number input with ParseIntPipe causes 400 error [OK]

Common Mistakes:

• Expecting string output instead of error
• Assuming NaN is returned
• Thinking it defaults to 0

5. You want to create a NestJS route that accepts a query parameter enabled which should be a boolean. Which of the following is the best way to ensure enabled is automatically converted and validated as a boolean?

hard

A. @Query('enabled', ParseIntPipe) enabled: boolean

B. @Query('enabled', ParseBoolPipe) enabled: boolean

C. @Body('enabled', ParseBoolPipe) enabled: string

D. @Param('enabled', ParseBoolPipe) enabled: boolean

Solution

1. Step 1: Identify the correct decorator for query parameters

   Query parameters use the @Query decorator.

2. Step 2: Match the pipe and parameter type

   ParseBoolPipe converts string to boolean, so parameter type must be boolean.

3. Final Answer:

   @Query('enabled', ParseBoolPipe) enabled: boolean -> Option B

4. Quick Check:

   Query param + ParseBoolPipe + boolean type = correct [OK]

Hint: Use @Query with ParseBoolPipe for boolean query params [OK]

Common Mistakes:

• Using ParseIntPipe for boolean values
• Using @Body or @Param instead of @Query for query params
• Mismatching parameter type with pipe output