typescript 53 lines · 8 steps

Wiring validation and Swagger docs in NestJS

A DTO's decorators drive both runtime validation and generated OpenAPI docs from one source of truth.

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NestJS Intermediate validation openapi decorators dto bootstrap
1import { NestFactory } from '@nestjs/core';
2import { DocumentBuilder, SwaggerModule } from '@nestjs/swagger';
3import { ValidationPipe } from '@nestjs/common';
4import { ApiProperty } from '@nestjs/swagger';
5import { IsEmail, IsInt, IsOptional, Min } from 'class-validator';
6import { Type } from 'class-transformer';
7import { AppModule } from './app.module';
8 
9export class CreateUserDto {
10 @ApiProperty({ example: 'ada@example.com', description: 'Unique login email' })
11 @IsEmail()
12 email: string;
13 
14 @ApiProperty({ example: 'Ada Lovelace', minLength: 2, maxLength: 120 })
15 fullName: string;
16 
17 @ApiProperty({ example: 34, minimum: 0, description: 'Age in whole years' })
18 @Type(() => Number)
19 @IsInt()
20 @Min(0)
21 age: number;
22 
23 @ApiProperty({
24 required: false,
25 enum: ['admin', 'member', 'guest'],
26 default: 'member',
27 })
28 @IsOptional()
29 role?: 'admin' | 'member' | 'guest';
30}
31 
32async function bootstrap() {
33 const app = await NestFactory.create(AppModule);
34 
35 app.useGlobalPipes(new ValidationPipe({ whitelist: true, transform: true }));
36 
37 const config = new DocumentBuilder()
38 .setTitle('Users API')
39 .setDescription('CRUD endpoints for managing platform users')
40 .setVersion('1.0.0')
41 .addBearerAuth()
42 .addTag('users')
43 .build();
44 
45 const document = SwaggerModule.createDocument(app, config);
46 SwaggerModule.setup('docs', app, document, {
47 swaggerOptions: { persistAuthorization: true },
48 });
49 
50 await app.listen(process.env.PORT ?? 3000);
51}
52 
53bootstrap();
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Decorators let one DTO class serve as both the validation contract and the API documentation.
  2. 2A global ValidationPipe with whitelist and transform strips unknown fields and coerces types automatically.
  3. 3Swagger's DocumentBuilder reads your decorators to produce a live, interactive API spec at runtime.

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