code: function sendfile(res, file, options, callback) {
    var done = false;
    var streaming;
  
    // request aborted
    function onaborted() {
      if (done) return;
      done = true;
  
      var err = new Error('Request aborted');
      err.code = 'ECONNABORTED';
      callback(err);
    }
  
    // directory
    function ondirectory() {
      if (done) return;
      done = true;
  
      var err = new Error('EISDIR, read');
      err.code = 'EISDIR';
      callback(err);
    }
  
    // errors
    function onerror(err) {
      if (done) return;
      done = true;
      callback(err);
    }
  
    // ended
    function onend() {
      if (done) return;
      done = true;
      callback();
    }
  
    // file
    function onfile() {
      streaming = false;
    }
  
    // finished
    function onfinish(err) {
      if (err && err.code === 'ECONNRESET') return onaborted();
      if (err) return onerror(err);
      if (done) return;
  
      setImmediate(function () {
        if (streaming !== false && !done) {
          onaborted();
          return;
        }
  
        if (done) return;
        done = true;
        callback();
      });
    }
  
    // streaming
    function onstream() {
      streaming = true;
    }
  
    file.on('directory', ondirectory);
    file.on('end', onend);
    file.on('error', onerror);
    file.on('file', onfile);
    file.on('stream', onstream);
    onFinished(res, onfinish);
  
    if (options.headers) {
      // set headers on successful transfer
      file.on('headers', function headers(res) {
        var obj = options.headers;
        var keys = Object.keys(obj);
  
        for (var i = 0; i < keys.length; i++) {
          var k = keys[i];
          res.setHeader(k, obj[k]);
        }
      });
    }
  
    // pipe
    file.pipe(res);
  }
docs: pipe the send file stream