Custom pipes in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this custom pipe transformation?

Consider this NestJS custom pipe that transforms input to uppercase. What will be the output when the pipe is applied to the string 'hello'?

NestJS

import { PipeTransform, Injectable, ArgumentMetadata } from '@nestjs/common';

@Injectable()
export class UppercasePipe implements PipeTransform {
  transform(value: any, metadata: ArgumentMetadata) {
    return typeof value === 'string' ? value.toUpperCase() : value;
  }
}

// Usage example:
// const result = new UppercasePipe().transform('hello', { type: 'body', metatype: String, data: '' });

A 'HELLO'

B 'hello'

C undefined

D Throws TypeError

💡 Hint

Think about what the transform method does to string inputs.

The pipe checks if the input is a string and converts it to uppercase. So 'hello' becomes 'HELLO'.

📝 Syntax

intermediate

Which option correctly implements a custom pipe that validates a positive integer?

You want to create a custom pipe that throws an error if the input is not a positive integer. Which code snippet correctly implements this?

A

import { PipeTransform, Injectable, BadRequestException } from '@nestjs/common';

@Injectable()
export class PositiveIntPipe implements PipeTransform {
  transform(value: any) {
    const val = parseFloat(value);
    if (!Number.isInteger(val) || val <= 0) {
      throw new BadRequestException('Value must be a positive integer');
    }
    return val;
  }
}

B

import { PipeTransform, Injectable } from '@nestjs/common';

@Injectable()
export class PositiveIntPipe implements PipeTransform {
  transform(value: any) {
    if (value <= 0) {
      throw new Error('Value must be positive');
    }
    return value;
  }
}

C

import { PipeTransform, Injectable, BadRequestException } from '@nestjs/common';

@Injectable()
export class PositiveIntPipe implements PipeTransform {
  transform(value: any) {
    const val = Number(value);
    if (val < 0) {
      throw new BadRequestException('Value must be positive');
    }
    return val;
  }
}

D

import { PipeTransform, Injectable, BadRequestException } from '@nestjs/common';

@Injectable()
export class PositiveIntPipe implements PipeTransform {
  transform(value: any) {
    const val = parseInt(value, 10);
    if (isNaN(val) || val <= 0) {
      throw new BadRequestException('Value must be a positive integer');
    }
    return val;
  }
}

💡 Hint

Check for integer parsing and proper error throwing.

Option D correctly parses the input as an integer, checks if it's positive, and throws the right exception. Others either miss integer check or throw generic errors.

🔧 Debug

advanced

Why does this custom pipe cause a runtime error?

This pipe is intended to trim string inputs. Why does it cause a runtime error when a number is passed?

NestJS

import { PipeTransform, Injectable } from '@nestjs/common';

@Injectable()
export class TrimPipe implements PipeTransform {
  transform(value: any) {
    return value.trim();
  }
}

A Because the transform method must return a Promise, but it returns a string.

B Because 'value' might not be a string and numbers do not have a trim method, causing a TypeError.

C Because the pipe is missing the @Injectable decorator, so it cannot be instantiated.

D Because the pipe does not handle null values, causing a ReferenceError.

💡 Hint

Think about what happens when you call trim on a non-string.

Calling trim() on a number causes a TypeError since numbers don't have that method. The pipe should check the type before calling trim.

❓ state_output

advanced

What is the value returned by this custom pipe after transformation?

Given this pipe that parses JSON strings, what is the returned value when input is '{"a":1,"b":2}'?

NestJS

import { PipeTransform, Injectable, BadRequestException } from '@nestjs/common';

@Injectable()
export class ParseJsonPipe implements PipeTransform {
  transform(value: any) {
    try {
      return JSON.parse(value);
    } catch {
      throw new BadRequestException('Invalid JSON string');
    }
  }
}

A {'a': 1, 'b': 2}

B {a:1,b:2}

C Throws BadRequestException

D {"a":1,"b":2}

💡 Hint

Remember what JSON.parse returns.

JSON.parse converts the JSON string into a JavaScript object with keys and values.

🧠 Conceptual

expert

Which statement about custom pipes in NestJS is true?

Select the correct statement about how custom pipes work in NestJS.

A Custom pipes can only be used for validation and cannot modify the input value.

B Custom pipes run after the controller method executes to modify the response.

C Custom pipes run before route handlers and can transform or validate incoming data.

D Custom pipes must always return a Promise and cannot return synchronous values.

💡 Hint

Think about when pipes are executed in the request lifecycle.

Pipes run before the route handler and can transform or validate data. They can return synchronous or asynchronous values.