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.