FileValidator

Kind: Class

Source: packages/common/pipes/file/file-validator.interface.ts

Part of: Common

Interface describing FileValidators, which can be added to a ParseFilePipe

FileValidator defines the contract for validators used by ParseFilePipe to inspect uploaded files. Implementations provide validation logic through isValid() and return a user-facing failure message through buildErrorMessage() when validation fails.

Methods

Diagram

mermaid
graph LR
  A[Uploaded file] --> B[ParseFilePipe]
  B --> C[FileValidator.isValid]
  C -->|Valid| D[Controller handler]
  C -->|Invalid| E[FileValidator.buildErrorMessage]
  E --> F[Validation error response]

Usage

ts
import {
  FileValidator,
  ParseFilePipe,
  UploadedFile,
  UseInterceptors,
  FileInterceptor,
  Controller,
  Post,
} from '@nestjs/common';

class AllowedMimeTypeValidator implements FileValidator {
  isValid(file?: Express.Multer.File): boolean {
    return file?.mimetype === 'image/png';
  }

  buildErrorMessage(): string {
    return 'Only PNG image uploads are allowed.';
  }
}

@Controller('uploads')
export class UploadController {
  @Post()
  @UseInterceptors(FileInterceptor('file'))
  upload(
    @UploadedFile(
      new ParseFilePipe({
        validators: [new AllowedMimeTypeValidator()],
      }),
    )
    file: Express.Multer.File,
  ) {
    return {
      filename: file.originalname,
      mimetype: file.mimetype,
    };
  }
}

AI Coding Instructions

• Implement both isValid() and buildErrorMessage() for every custom FileValidator.
• Keep isValid() focused on validation and return either a boolean or Promise<boolean> for asynchronous checks.
• Return clear, client-safe messages from buildErrorMessage(); avoid exposing internal storage or security details.
• Register validator instances in ParseFilePipe through its validators option.
• Account for missing files in validators when the upload field may be optional.