Built-in pipes (ParseIntPipe, ParseBoolPipe) in NestJS

Introduction

Pipes help convert and validate data in NestJS. ParseIntPipe and ParseBoolPipe make sure your data is the right type, like numbers or true/false.

When you get a number as a string from a URL and want to use it as a number in your code.

When you receive a true/false value as a string and want to use it as a boolean.

When you want to avoid manual checks and conversions for common data types in your route handlers.

Syntax

NestJS

import { ParseIntPipe, ParseBoolPipe } from '@nestjs/common';

@Get(':id')
getById(@Param('id', ParseIntPipe) id: number) {
  // id is a number here
}

@Get('active/:status')
getByStatus(@Param('status', ParseBoolPipe) status: boolean) {
  // status is a boolean here
}

Use pipes inside parameter decorators like @Param(), @Query(), or @Body() to transform input data.

If the input cannot be converted, NestJS will automatically return a 400 error.

Examples

This converts the 'id' parameter from string to number automatically.

NestJS

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

This converts the 'status' parameter from string to boolean (true or false).

NestJS

@Get('active/:status')
getActive(@Param('status', ParseBoolPipe) status: boolean) {
  return `Active status is ${status}`;
}

ParseIntPipe can also be used with query parameters.

NestJS

@Get('query')
getQuery(@Query('page', ParseIntPipe) page: number) {
  return `Page number is ${page}`;
}

Sample Program

This controller has two routes. One converts the 'id' param to a number. The other converts 'status' param to a boolean. If conversion fails, NestJS sends a 400 error automatically.

NestJS

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

@Controller('items')
export class ItemsController {
  @Get(':id')
  getItemById(@Param('id', ParseIntPipe) id: number) {
    return `Item ID is ${id}`;
  }

  @Get('available/:status')
  getItemAvailability(@Param('status', ParseBoolPipe) status: boolean) {
    return `Item availability is ${status}`;
  }
}

Important Notes

ParseIntPipe converts strings to integers using parseInt(), which truncates decimals (e.g., '3.14' becomes 3) and rejects non-numeric inputs with an error.

ParseBoolPipe accepts 'true', 'false', '1', '0' (case insensitive) as valid boolean strings.

You can customize error messages by extending these pipes if needed.

Summary

ParseIntPipe and ParseBoolPipe automatically convert string inputs to number and boolean types.

They help keep your code clean by avoiding manual parsing and validation.

Invalid inputs cause NestJS to return a 400 error, improving API reliability.

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