How do you implement data validation for query parameters in Nest.js routes?

In Nest.js, implementing query parameter data validation typically follows a structured approach that effectively enhances the robustness and maintainability of the code. Nest.js uses classes and decorators to handle HTTP requests and can be combined with powerful class validators such as class-validator to validate query parameters. The following is a specific implementation step:

Step 1: Install Dependencies

First, ensure that you have installed class-validator and class-transformer libraries. If not installed, you can install them using the following command:

bash
npm install class-validator class-transformer

Step 2: Create DTO (Data Transfer Object)

DTO (Data Transfer Object) is used to encapsulate data and validate it using class validators. To validate query parameters, create a dedicated DTO class. For example, suppose you have an API to retrieve a user list that requires validating the incoming age and name query parameters:

typescript
import { IsInt, IsOptional, IsString, Min } from 'class-validator';

export class UserQueryDto {
  @IsOptional()
  @IsInt()
  @Min(0)
  age?: number;

  @IsOptional()
  @IsString()
  name?: string;
}

In the UserQueryDto class, we define two properties age and name, and apply decorators from class-validator to set validation rules. @IsOptional() indicates that these parameters are optional, @IsInt() and @IsString() validate data types, and @Min(0) ensures age is non-negative.

Step 3: Use DTO in the Controller

In the Nest.js controller, use the UserQueryDto defined above to retrieve and validate query parameters. Implement this using the @Query() decorator combined with pipes:

typescript
import { Controller, Get, Query } from '@nestjs/common';
import { UserQueryDto } from './dto/user-query.dto';
import { ParseIntPipe, ValidationPipe } from '@nestjs/common';

@Controller('users')
export class UserController {
  @Get()
  findAll(@Query(new ValidationPipe({ transform: true })) query: UserQueryDto) {
    // Use query.age and query.name for subsequent operations
    return `Fetching users with age: ${query.age} and name: ${query.name}`;
  }
}

In this code, @Query(new ValidationPipe({ transform: true })) handles and validates incoming query parameters. The { transform: true } option ensures incoming query parameters are correctly converted to the data types defined in UserQueryDto.

Summary

By combining DTOs with class validators, implementing query parameter data validation in Nest.js not only ensures data correctness but also improves code readability and maintainability. This approach is particularly suitable for managing and maintaining various input validation rules when building complex applications.