docs: function validChunk(stream, state, chunk, cb) {
    var valid = true;
    if (!Buffer.isBuffer(chunk) &&
        'string' !== typeof chunk &&
        chunk !== null &&
        chunk !== undefined &&
        !state.objectMode) {
      var er = new TypeError('Invalid non-string/buffer chunk');
      stream.emit('error', er);
      timers.setImmediate(function() {
        cb(er);
      });
      valid = false;
    }
    return valid;
  }