Built-in pipes (ParseIntPipe, ParseBoolPipe) in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the ParseIntPipe in NestJS?

ParseIntPipe converts a string input from a request into an integer. It ensures the value is a valid number and throws an error if not.

beginner

How does ParseBoolPipe interpret string values?

ParseBoolPipe converts string inputs like 'true' or 'false' (case insensitive) into boolean true or false. It throws an error for invalid boolean strings.

beginner

Show a simple example of using ParseIntPipe in a controller route parameter.

In a controller method, use @Param('id', ParseIntPipe) id: number to automatically convert the 'id' parameter to a number.

intermediate

What happens if ParseIntPipe receives a non-numeric string?

It throws a BadRequestException, causing NestJS to respond with a 400 error indicating invalid input.

intermediate

Why use built-in pipes like ParseIntPipe and ParseBoolPipe instead of manual conversion?

They simplify code by automatically validating and converting inputs, improving reliability and reducing boilerplate error checks.

What does ParseIntPipe do in NestJS?

A Converts string input to an integer

B Converts boolean input to string

C Parses JSON objects

D Encrypts input data

ParseIntPipe converts string inputs to integers and validates them.

Which string value will ParseBoolPipe convert to false?

A "true"

B "false"

C "yes"

D "1"

ParseBoolPipe converts 'false' (case insensitive) to boolean false.

What error does ParseIntPipe throw on invalid input?

A InternalServerErrorException

B UnauthorizedException

C NotFoundException

D BadRequestException

ParseIntPipe throws BadRequestException for invalid integer input.

How do you apply ParseIntPipe to a route parameter named 'id'?

A @Param('id', ParseIntPipe) id: number

B @Body('id', ParseIntPipe) id: number

C @Query('id', ParseIntPipe) id: number

D @Headers('id', ParseIntPipe) id: number

Use @Param decorator with ParseIntPipe to convert route parameters.

Why prefer built-in pipes over manual parsing?

A They add extra features like caching

B They slow down the app for security

C They automatically validate and convert inputs

D They replace database queries

Built-in pipes simplify input validation and conversion, reducing errors.

Explain how ParseIntPipe works and when you would use it in a NestJS controller.

Think about converting URL parameters to numbers safely.

Describe the behavior of ParseBoolPipe and how it handles different string inputs.

Consider how to safely get true/false from user input.

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