src/nestjs/BufferResponseInterceptor.ts

import {
  CallHandler,
  ExecutionContext,
  Injectable,
  NestInterceptor,
  StreamableFile,
} from '@nestjs/common';
import { Request, Response } from 'express';
import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';

import { Base64File } from '../structures/files.dto';

@Injectable()
/**
 * Interceptor to convert Buffer response to Base64File or StreamableFile
 * based on Accept header.
 * It should go together with @ApiFileAcceptHeader in controller method
 * So we have proper Swagger documentation
 */
export class BufferResponseInterceptor<T>
  implements NestInterceptor<Buffer, Base64File | StreamableFile>
{
  constructor(
    private mimetype: string,
    private filename: string = null,
  ) {}

  intercept(
    context: ExecutionContext,
    next: CallHandler,
  ): Observable<Base64File | StreamableFile> {
    const ctx = context.switchToHttp();
    const request = ctx.getRequest<Request>();
    const response = ctx.getResponse<Response>();

    return next
      .handle()
      .pipe(map((buffer) => this.processBuffer(buffer, request, response)));
  }

  processBuffer(buffer: Buffer, request: Request, response: Response) {
    // Check buffer is Buffer
    if (!Buffer.isBuffer(buffer)) {
      return buffer;
    }

    const accept = request.headers['accept'];
    if (accept == 'application/json') {
      return {
        mimetype: this.mimetype,
        data: buffer.toString('base64'),
      };
    }
    const file = new StreamableFile(buffer);
    response.set({
      'Content-Type': this.mimetype,
      'Content-Length': buffer.length,
    });
    if (this.filename) {
      response.set({
        'Content-Disposition': `attachment; filename="${this.filename}"`,
      });
    }
    return file;
  }
}