Built-in pipes (ParseIntPipe, ParseBoolPipe) in NestJS - Mini Project: Build & Apply

Using Built-in Pipes ParseIntPipe and ParseBoolPipe in NestJS

📖 Scenario: You are building a simple NestJS controller to handle user requests where some query parameters need to be converted to specific types automatically.For example, a user might send a query parameter age as a string, but you want it as a number. Similarly, a query parameter active might come as a string but should be treated as a boolean.

🎯 Goal: Create a NestJS controller with two GET routes that use the built-in pipes ParseIntPipe and ParseBoolPipe to automatically convert query parameters to the correct types.This will help ensure your controller methods receive the right data types without manual conversion.

📋 What You'll Learn

Create a controller class named UserController

Add a GET route /user/age that accepts a query parameter age and uses ParseIntPipe to convert it to a number

Add a GET route /user/active that accepts a query parameter active and uses ParseBoolPipe to convert it to a boolean

Use NestJS decorators @Controller, @Get, and @Query properly

Import ParseIntPipe and ParseBoolPipe from @nestjs/common

💡 Why This Matters

🌍 Real World

Web APIs often receive query parameters as strings. Using pipes like ParseIntPipe and ParseBoolPipe helps convert these strings to the correct types automatically, reducing manual parsing and errors.

💼 Career

Understanding and using NestJS built-in pipes is essential for backend developers working with NestJS to build robust and type-safe APIs.

1

Create the UserController class with imports

Create a NestJS controller class named UserController. Import Controller, Get, and Query from @nestjs/common. Do not add any methods yet.

NestJS

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

// Create the UserController class with @Controller decorator
// Your code here

Hint

Use @Controller('user') above the class declaration to set the route prefix.

2

Add ParseIntPipe import and age route method

Import ParseIntPipe from @nestjs/common. Add a GET route method named getAge with path 'age'. Use @Query('age', ParseIntPipe) to get the age query parameter as a number. The method should return the age value.

NestJS

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

@Controller('user')
export class UserController {
  // Add getAge method below
  // Your code here
}

Hint

Use @Get('age') above the method and @Query('age', ParseIntPipe) to convert the query parameter.

3

Add ParseBoolPipe import and active route method

Import ParseBoolPipe from @nestjs/common. Add a GET route method named getActive with path 'active'. Use @Query('active', ParseBoolPipe) to get the active query parameter as a boolean. The method should return the active value.

NestJS

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

@Controller('user')
export class UserController {
  @Get('age')
  getAge(@Query('age', ParseIntPipe) age: number): number {
    return age;
  }

  // Add getActive method below
  // Your code here
}

Hint

Use @Get('active') and @Query('active', ParseBoolPipe) to convert the query parameter to boolean.

4

Complete the UserController with both routes

Ensure the UserController class includes both getAge and getActive methods with their respective pipes and returns. The class should be decorated with @Controller('user') and import all necessary decorators and pipes from @nestjs/common.

NestJS

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

// Confirm the UserController class is complete with both methods
// Your code here

Hint

Make sure both methods and imports are present and correctly used.

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