Built-in pipes (ParseIntPipe, ParseBoolPipe) in NestJS - Step-by-Step Execution

Concept Flow - Built-in pipes (ParseIntPipe, ParseBoolPipe)

Receive Request Parameter as String

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Apply Pipe: ParseIntPipe or ParseBoolPipe

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Valid Conversion

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Pass Converted Value

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Controller Method Executes

The pipe takes a string input from a request, tries to convert it to int or bool, passes it if valid, or throws an error if invalid.

Execution Sample

NestJS

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

This code converts the 'id' route parameter from string to number using ParseIntPipe before using it.

Execution Table

Step	Input Parameter	Pipe Applied	Conversion Result	Action Taken	Output or Error
1	'123'	ParseIntPipe	123 (number)	Pass converted value	`ID is 123`
2	'abc'	ParseIntPipe	NaN (invalid)	Throw BadRequestException	400 Bad Request Error
3	'true'	ParseBoolPipe	true (boolean)	Pass converted value	true
4	'false'	ParseBoolPipe	false (boolean)	Pass converted value	false
5	'yes'	ParseBoolPipe	Invalid boolean	Throw BadRequestException	400 Bad Request Error

💡 Execution stops when conversion fails and pipe throws BadRequestException.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
inputParam	undefined	'123'	'abc'	'true'	'false'	'yes'
convertedValue	undefined	123	NaN	true	false	Invalid
action	none	pass	throw error	pass	pass	throw error

Key Moments - 2 Insights

Why does ParseIntPipe throw an error when the input is 'abc'?

Because 'abc' cannot be converted to a number, the pipe detects NaN and throws a BadRequestException as shown in execution_table step 2.

What happens if ParseBoolPipe receives 'yes' as input?

ParseBoolPipe only accepts 'true' or 'false' strings (case insensitive). 'yes' is invalid, so it throws a BadRequestException as in execution_table step 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the output when input parameter is '123' with ParseIntPipe?

A `ID is 123`

B 400 Bad Request Error

C NaN

D undefined

💡 Hint

Check execution_table row 1 under Output or Error column.

At which step does the condition cause a BadRequestException due to invalid boolean?

A Step 3

B Step 2

C Step 5

D Step 4

💡 Hint

Look at execution_table rows with ParseBoolPipe and check Conversion Result and Action Taken.

If the input parameter changes from 'false' to 'true' with ParseBoolPipe, what changes in variable_tracker?

A inputParam changes from true to false

B convertedValue changes from false to true

C action changes from throw error to pass

D No change in variables

💡 Hint

Check variable_tracker rows for convertedValue at After Step 3 and After Step 4.

Concept Snapshot

Built-in pipes like ParseIntPipe and ParseBoolPipe convert string inputs from requests into numbers or booleans.
If conversion succeeds, the converted value is passed to the controller.
If conversion fails, a BadRequestException is thrown, stopping execution.
Use pipes in @Param or @Query decorators to validate and transform inputs automatically.

Full Transcript

In NestJS, built-in pipes such as ParseIntPipe and ParseBoolPipe help convert string inputs from HTTP requests into numbers or booleans. When a request comes in, the pipe tries to convert the string parameter. If successful, it passes the converted value to the controller method. If the conversion fails, the pipe throws a BadRequestException, which results in a 400 error response. For example, ParseIntPipe converts '123' to number 123, but throws an error for 'abc'. ParseBoolPipe converts 'true' or 'false' strings to booleans, but throws an error for invalid strings like 'yes'. Using these pipes ensures your controller receives correctly typed data and handles invalid inputs gracefully.