How to Fix Validation Error in NestJS

Validation Errors in NestJS

NestJS leverages class-validator decorators for DTO validation. Errors occur when incoming data violates these constraints and the ValidationPipe rejects the request.

Typical Triggers

• Missing required fields in request body
• String values where numbers are expected
• Nested objects failing their own validation rules

How to Fix

Configure the ValidationPipe and write clear DTOs:

// dto/create-product.dto.ts
import { IsString, IsNumber, Min, MaxLength, IsOptional } from 'class-validator';

export class CreateProductDto {
  @IsString()
  @MaxLength(200)
  name: string;

  @IsNumber({ maxDecimalPlaces: 2 })
  @Min(0.01)
  price: number;

  @IsOptional()
  @IsString()
  @MaxLength(1000)
  description?: string;
}

// main.ts — configure globally
app.useGlobalPipes(new ValidationPipe({
  whitelist: true,
  forbidNonWhitelisted: true,
  transform: true,
  exceptionFactory: (errors) => {
    const messages = errors.map(e =>
      Object.values(e.constraints || {}).join(', ')
    );
    return new BadRequestException(messages);
  },
}));

The whitelist option strips unknown properties, and forbidNonWhitelisted rejects them — preventing unexpected data from reaching your handlers.

Prevention Tips

To avoid this issue recurring, add automated checks to your CI/CD pipeline. Write integration tests that exercise the failure path — not just the happy path. Use linting rules to enforce best practices across your team. Consider adding health checks that detect this class of error early in staging before it reaches production.

Bugsly for NestJS

Bugsly captures validation pipe rejections with the DTO class name and all failed constraints, helping you see which endpoints receive the most invalid requests and from which clients.