Pipe binding (parameter, method, controller, global) in NestJS

Introduction

Pipes help process and validate data before it reaches your code. Binding pipes lets you control where and how this happens.

When you want to check or change data for a single parameter in a request.

When you want to apply data validation or transformation to all parameters in a method.

When you want all methods in a controller to use the same data processing rules.

When you want to apply a pipe globally to all incoming requests in your app.

Syntax

NestJS

1. Parameter binding:
@Get(':id')
method(@Param('id', Pipe) id: string) {}

2. Method binding:
@UsePipes(Pipe)
method() {}

3. Controller binding:
@UsePipes(Pipe)
@Controller()
class MyController {}

4. Global binding:
app.useGlobalPipes(new Pipe());

Use @UsePipes() decorator for method and controller binding.

Parameter binding applies pipe only to that parameter.

Examples

This converts the 'id' parameter to a number using ParseIntPipe.

NestJS

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

This validates the whole body data for the create method.

NestJS

@UsePipes(ValidationPipe)
@Post()
create(@Body() data: CreateDto) {
  return data;
}

All methods in UserController will validate input data.

NestJS

@UsePipes(ValidationPipe)
@Controller('users')
class UserController {
  @Post()
  create(@Body() data: CreateUserDto) {
    return data;
  }
}

This applies validation to all requests in the app.

NestJS

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

Sample Program

This example shows pipe binding at all levels: parameter (ParseIntPipe), controller (ValidationPipe), and global (ValidationPipe in main).

NestJS

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

@Controller('items')
@UsePipes(new ValidationPipe())
export class ItemsController {
  @Get(':id')
  getItem(@Param('id', ParseIntPipe) id: number) {
    return { itemId: id };
  }
}

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

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

Important Notes

Pipes run in this order: global, controller, method, then parameter.

Parameter pipes override method or controller pipes for that parameter.

Use pipes to keep your code clean by handling validation and transformation outside your main logic.

Summary

Pipes process data before your code uses it.

You can bind pipes to parameters, methods, controllers, or globally.

Binding location controls how widely the pipe applies.

Practice

1. What is the main purpose of binding a pipe to a parameter in NestJS?

easy

A. To define routing paths for the controller methods

B. To apply middleware logic before the controller handles the request

C. To globally change all incoming requests in the application

D. To transform or validate the specific parameter before the method uses it

Solution

1. Step 1: Understand parameter-level pipe binding

   Binding a pipe to a parameter means the pipe runs only on that parameter to transform or validate it before the method uses it.

2. Step 2: Differentiate from other bindings

   Middleware and global pipes affect broader scopes, not just a single parameter. Routing is unrelated to pipes.

3. Final Answer:

   To transform or validate the specific parameter before the method uses it -> Option D

4. Quick Check:

   Parameter pipe = transform/validate parameter [OK]

Hint: Parameter pipes affect only one input value [OK]

Common Mistakes:

• Confusing parameter pipes with global pipes
• Thinking pipes define routes
• Mixing middleware with pipes

2. Which of the following is the correct syntax to bind a pipe globally in a NestJS application?

easy

A. @UsePipes(new ValidationPipe()) on a controller class

B. app.useGlobalPipes(new ValidationPipe());

C. @UsePipes(new ValidationPipe()) on a method

D. Bind pipe inside a method parameter decorator

Solution

1. Step 1: Identify global pipe binding syntax

   Global pipes are bound using the app instance with app.useGlobalPipes(pipeInstance).

2. Step 2: Differentiate from controller and method bindings

   @UsePipes decorator applies pipes at controller or method level, not globally.

3. Final Answer:

   app.useGlobalPipes(new ValidationPipe()); -> Option B

4. Quick Check:

   Global pipe = app.useGlobalPipes() [OK]

Hint: Global pipes use app.useGlobalPipes() method [OK]

Common Mistakes:

• Using @UsePipes for global binding
• Binding pipes inside method parameters for global effect
• Confusing middleware with pipes

3. Given the following code, what will be the output if the incoming request has a query parameter age='25' (string)?

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

@Controller('users')
export class UserController {
  @Get()
  getUser(@Query('age', ParseIntPipe) age: number) {
    return { age, type: typeof age };
  }
}

medium

A. Runtime error due to type mismatch

B. {"age": "25", "type": "string"}

C. {"age": 25, "type": "number"}

D. {"age": null, "type": "object"}

Solution

1. Step 1: Understand ParseIntPipe effect on query parameter

   ParseIntPipe converts the string '25' to the number 25 before passing it to the method.

2. Step 2: Check returned object values

   The method returns an object with age as number 25 and type as 'number'.

3. Final Answer:

   {"age": 25, "type": "number"} -> Option C

4. Quick Check:

   ParseIntPipe converts string to number [OK]

Hint: ParseIntPipe converts string query to number [OK]

Common Mistakes:

• Assuming age remains string
• Expecting runtime error on conversion
• Confusing type of returned value

4. Identify the error in this NestJS controller code snippet where a pipe is bound at the method level:

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

@Controller('products')
export class ProductController {
  @Get()
  @UsePipes(ValidationPipe)
  findProduct(@Query('id') id: string) {
    return id;
  }
}

medium

A. ValidationPipe should be instantiated with 'new' keyword in @UsePipes

B. @UsePipes cannot be used on methods, only on controllers

C. Query decorator is missing a pipe binding

D. Controller decorator is missing parameters

Solution

1. Step 1: Check @UsePipes decorator usage

   @UsePipes expects an instance of a pipe, so ValidationPipe must be called with 'new'.

2. Step 2: Validate other parts

   @UsePipes can be used on methods, Query decorator is valid, Controller decorator is correct.

3. Final Answer:

   ValidationPipe should be instantiated with 'new' keyword in @UsePipes -> Option A

4. Quick Check:

   @UsePipes needs pipe instances [OK]

Hint: Always instantiate pipes with 'new' in decorators [OK]

Common Mistakes:

• Passing pipe class instead of instance
• Thinking @UsePipes only works on controllers
• Missing pipe on parameter is not an error

5. You want to apply a custom pipe that trims whitespace from all string inputs in a controller and also validate numbers globally. Which binding strategy is best to achieve this cleanly?

hard

A. Bind the trim pipe at controller level and the number validation pipe globally

B. Bind both pipes globally to affect all controllers and methods

C. Bind both pipes only on method parameters where needed

D. Bind the trim pipe globally and the number validation pipe at method level

Solution

1. Step 1: Understand scope of pipes for separation of concerns

   Trimming strings is specific to this controller's inputs, so controller-level binding keeps it scoped and clean.

2. Step 2: Apply number validation globally

   Number validation is common across the app, so global binding ensures consistent validation everywhere.

3. Final Answer:

   Bind the trim pipe at controller level and the number validation pipe globally -> Option A

4. Quick Check:

   Controller pipe for specific, global pipe for common validation [OK]

Hint: Use controller pipes for specific, global pipes for common tasks [OK]

Common Mistakes:

• Binding all pipes globally causing unnecessary processing
• Binding all pipes only on parameters causing repetition
• Mixing global and method-level without clear reason