@loopback/rest/src/writer.ts

// Copyright IBM Corp. and LoopBack contributors 2017,2020. All Rights Reserved.
// Node module: @loopback/rest
// This file is licensed under the MIT License.
// License text available at https://opensource.org/licenses/MIT

import {Readable} from 'stream';
import {OperationRetval, Response} from './types';

/**
 * Writes the result from Application controller method
 * into the HTTP response
 *
 * @param response - HTTP Response
 * @param result - Result from the API to write into HTTP Response
 */
export function writeResultToResponse(
  // not needed and responsibility should be in the sequence.send
  response: Response,
  // result returned back from invoking controller method
  result: OperationRetval,
): void {
  // Bypass response writing if the controller method returns `response` itself
  // or the response headers have been sent
  if (result === response || response.headersSent) {
    return;
  }
  if (result === undefined) {
    response.statusCode = 204;
    response.end();
    return;
  }

  const isStream =
    result instanceof Readable || typeof result?.pipe === 'function';

  if (isStream) {
    response.setHeader('Content-Type', 'application/octet-stream');
    // Stream
    result.pipe(response);
    return;
  }
  switch (typeof result) {
    case 'object':
    case 'boolean':
    case 'number':
      if (Buffer.isBuffer(result)) {
        // Buffer for binary data
        response.setHeader('Content-Type', 'application/octet-stream');
      } else {
        // TODO(ritch) remove this, should be configurable
        // See https://github.com/loopbackio/loopback-next/issues/436
        response.setHeader('Content-Type', 'application/json');
        // TODO(bajtos) handle errors - JSON.stringify can throw
        result = JSON.stringify(result);
      }
      break;
    default:
      response.setHeader('Content-Type', 'text/plain');
      result = result.toString();
      break;
  }
  response.end(result);
}