Pipe binding (parameter, method, controller, global) in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when a pipe is bound at the parameter level?

Consider a NestJS controller method where a custom pipe is applied only to a single parameter. What will be the effect of this pipe binding?

NestJS

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

@Controller('items')
export class ItemsController {
  @Get(':id')
  getItem(@Param('id', ParseIntPipe) id: number) {
    return { id, type: typeof id };
  }
}

A The pipe is ignored because it must be applied at the controller level to work.

B The pipe converts the 'id' parameter to a number only for this method call, so the returned type is 'number'.

C The pipe converts all parameters named 'id' in all controllers to number globally.

D The pipe converts the 'id' parameter to a string instead of a number.

💡 Hint

Think about where the pipe is applied and what it affects.

When a pipe is bound at the parameter level, it only affects that specific parameter in that method. Here, ParseIntPipe converts the 'id' string parameter to a number before the method uses it.

❓ component_behavior

intermediate

What happens when a pipe is bound at the controller level?

Given a NestJS controller with a pipe applied at the controller level, how does this pipe affect incoming requests?

NestJS

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

@UsePipes(ParseIntPipe)
@Controller('users')
export class UsersController {
  @Get(':id')
  getUser(@Param('id') id: number) {
    return { id, type: typeof id };
  }

  @Get('profile/:id')
  getProfile(@Param('id') id: number) {
    return { profileId: id, type: typeof id };
  }
}

A The ParseIntPipe is applied to all parameters in all methods of this controller, converting them to numbers.

B The ParseIntPipe is applied only to the first method in the controller.

C The ParseIntPipe is ignored because it must be applied globally to work.

D The ParseIntPipe converts all string parameters to uppercase strings.

💡 Hint

Consider the scope of the @UsePipes decorator when applied to a controller.

When a pipe is bound at the controller level using @UsePipes, it applies to all route handler methods within that controller. Here, ParseIntPipe converts all parameters to numbers for every method.

❓ component_behavior

advanced

What is the effect of binding a pipe globally in NestJS?

In a NestJS application, a pipe is bound globally in the main.ts file. How does this affect the request handling?

NestJS

import { ValidationPipe } from '@nestjs/common';
import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.useGlobalPipes(new ValidationPipe());
  await app.listen(3000);
}
bootstrap();

A The ValidationPipe runs only on parameters decorated with @UsePipes(ValidationPipe).

B The ValidationPipe runs only on controllers that explicitly use @UsePipes(ValidationPipe).

C The ValidationPipe runs only on methods decorated with @UsePipes(ValidationPipe).

D The ValidationPipe runs on every incoming request across all controllers and routes, validating inputs globally.

💡 Hint

Think about what 'global' means in this context.

Binding a pipe globally with app.useGlobalPipes applies it to every request in the app, regardless of controller or method. This ensures consistent validation or transformation everywhere.

📝 Syntax

advanced

Which option correctly applies a custom pipe only to a specific method in a NestJS controller?

Choose the correct way to apply a custom pipe named MyCustomPipe only to the create method of a controller.

NestJS

import { Controller, Post, Body, UsePipes } from '@nestjs/common';
import { MyCustomPipe } from './my-custom.pipe';

@Controller('products')
export class ProductsController {
  @Post()
  create(@Body() data: any) {
    return data;
  }
}

A

@Post()
create(@Body(MyCustomPipe) data: any) {
  return data;
}

B

@UsePipes(MyCustomPipe)
@Controller('products')
@Post()
create(@Body() data: any) {
  return data;
}

C

@Post()
@UsePipes(MyCustomPipe)
create(@Body() data: any) {
  return data;
}

D

@Controller('products')
@UsePipes(MyCustomPipe)
@Post()
create(@Body() data: any) {
  return data;
}

💡 Hint

Remember where to place the @UsePipes decorator to affect only one method.

To apply a pipe only to a specific method, place @UsePipes above that method. Option C correctly applies MyCustomPipe only to the create method.

🔧 Debug

expert

Why does this global pipe not transform input as expected?

Given the following NestJS setup, why does the global pipe not transform the incoming string to a number?

NestJS

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

@Injectable()
export class ToNumberPipe implements PipeTransform {
  transform(value: any, metadata: ArgumentMetadata) {
    return Number(value);
  }
}

// main.ts
import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.useGlobalPipes(new ToNumberPipe());
  await app.listen(3000);
}
bootstrap();

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

@Controller('orders')
export class OrdersController {
  @Get(':id')
  getOrder(@Param('id') id: number) {
    return { id, type: typeof id };
  }
}

A The global pipe runs but does not transform because the parameter decorator @Param does not trigger transformation by default.

B The pipe throws a runtime error because Number(value) is invalid for string inputs.

C The pipe is never called because it is not bound at the controller or method level.

D The pipe transforms the value correctly, so the type of id is 'number'.

💡 Hint

Consider how NestJS handles parameter transformation and when pipes run.

Global pipes run on the entire request body by default but do not automatically transform route parameters unless the pipe is explicitly set to transform parameters or the parameter decorator supports it. Here, @Param does not trigger transformation, so the id remains a string.