Reads express.js response

collect-express-response.ts

import { Response } from "express";

type WriteCallback = (error: Error | null | undefined) => void;

type WriteArguments1 = [chunk: any, callback?: WriteCallback];

type WriteArguments2 = [chunk: any, encoding: BufferEncoding, callback?: (error: Error | null | undefined) => void];

type EndCallback = () => void;

type EndParameters1 = [cb?: EndCallback];

type EndParameters2 = [chunk: any, cb?: EndCallback];

type EndParameters3 = [chunk: any, encoding: BufferEncoding, cb?: EndCallback];

export function collectResponseBody(res: Response): Promise<Buffer> {
  return new Promise<Buffer>((resolve, reject) => {
    const chunks: Buffer[] = [];
    const _write = res.write.bind(res);
    const _end = res.end.bind(res);

    res.write = function write(...args: WriteArguments1 | WriteArguments2) {
      const chunk = args[0];
      const encoding = args[1];
      const callback = args[2];
      return typeof encoding === "object" ? _write(chunk, encoding, callback) : _write(chunk, callback);
    };

    res.end = function end(...args: EndParameters1 | EndParameters2 | EndParameters3) {
      const chunk = args[0];
      const encoding = args[1];
      const callback = args[2];
      if (chunk) chunks.push(chunk);
      return typeof encoding === "object" ? _end(chunk, encoding, callback) : _end(chunk, callback);
    };

    res.once("error", (error) => reject(error));
    res.once("close", () => resolve(Buffer.concat(chunks)));
  });
}