How to validate GET parameters in NestJS

This is a short post on how to validate GET parameters in NestJS APIs using class-validator package and ValidationPipe. We will create a simple API that accepts 2 query parameters and validates them.

Prerequisites

• NestJS project installed with nest new command
• class-validator and class-transformer packages installed
• ValidationPipe set up in main.ts file

More detailed instructions on how to set up a new NestJS project for this post can be found by this link

Query parameters validation with DTO

First, we need to create a DTO (Data Transfer Object) that will specify the structure of our query parameters and their validation rules. Let's create a new file src/dto/get-request.dto.ts:

import { IsString, IsInt, IsNotEmpty } from 'class-validator';
import { Transform } from 'class-transformer';

export class GetRequestDto {
  @IsNotEmpty()
  @IsInt()
  @Transform(({ value }) => parseInt(value, 10))
  param1: number;

  @IsNotEmpty()
  @IsString()
  param2: string;
}

We can use this DTO in our app.controller.ts now:

import { Query, Controller, Get } from '@nestjs/common';
import { GetRequestDto } from './dto/get-request.dto';

@Controller()
export class AppController {
  @Get('/get')
  getRequest(@Query() query: GetRequestDto): GetRequestDto {
    return query;
  }
}

Please mention that we are using @Query decorator to get the query parameters and we are passing GetRequestDto as a parameter to the getRequest method. This will make NestJS to validate the query parameters according to the rules specified in GetRequestDto class.

We also need to transform the param1 value to a number, because query parameters are always strings. We can use class-transformer package to do this. We are using @Transform decorator to transform the value to a number.

Now, when we make a GET request to /get endpoint with query parameters param1 and param2, they will be validated. If the validation fails, NestJS will return a 400 Bad Request response with a detailed error message.