code: function respond(ctx) {
    // allow bypassing koa
    if (false === ctx.respond) return;
  
    const res = ctx.res;
    if (!ctx.writable) return;
  
    let body = ctx.body;
    const code = ctx.status;
  
    // ignore body
    if (statuses.empty[code]) {
      // strip headers
      ctx.body = null;
      return res.end();
    }
  
    if ("HEAD" == ctx.method) {
      if (!res.headersSent && isJSON(body)) {
        ctx.length = Buffer.byteLength(JSON.stringify(body));
      }
      return res.end();
    }
  
    // status body
    if (null == body) {
      body = ctx.message || String(code);
      if (!res.headersSent) {
        ctx.type = "text";
        ctx.length = Buffer.byteLength(body);
      }
      return res.end(body);
    }
  
    // responses
    if (Buffer.isBuffer(body)) return res.end(body);
    if ("string" == typeof body) return res.end(body);
    if (body instanceof Stream) return body.pipe(res);
  
    // body: json
    body = JSON.stringify(body);
    if (!res.headersSent) {
      ctx.length = Buffer.byteLength(body);
    }
    res.end(body);
  }
docs: Response helper.
  copied from the original