How to use the core-util-is.isBuffer function in core-util-is

function writeOrBuffer(stream, state, chunk, encoding, cb) {
  chunk = decodeChunk(state, chunk, encoding);
  if (util.isBuffer(chunk))
    encoding = 'buffer';
  var len = state.objectMode ? 1 : chunk.length;

  state.length += len;

  var ret = state.length < state.highWaterMark;
  // we must ensure that previous needDrain will not be reset to false.
  if (!ret)
    state.needDrain = true;

  if (state.writing || state.corked)
    state.buffer.push(new WriteReq(chunk, encoding, cb));
  else
    doWrite(stream, state, false, len, chunk, encoding, cb);

  return ret;

function validChunk(stream, state, chunk, cb) {
  var valid = true;
  if (!util.isBuffer(chunk) &&
      !util.isString(chunk) &&
      !util.isNullOrUndefined(chunk) &&
      !state.objectMode) {
    var er = new TypeError('Invalid non-string/buffer chunk');
    stream.emit('error', er);
    process.nextTick(function() {
      cb(er);
    });
    valid = false;
  }
  return valid;
}

Writable.prototype.write = function(chunk, encoding, cb) {
  var state = this._writableState;
  var ret = false;

  if (util.isFunction(encoding)) {
    cb = encoding;
    encoding = null;
  }

  if (util.isBuffer(chunk))
    encoding = 'buffer';
  else if (!encoding)
    encoding = state.defaultEncoding;

  if (!util.isFunction(cb))
    cb = function() {};

  if (state.ended)
    writeAfterEnd(this, state, cb);
  else if (validChunk(this, state, chunk, cb)) {
    state.pendingcb++;
    ret = writeOrBuffer(this, state, chunk, encoding, cb);
  }

  return ret;
};

function chunkInvalid(state, chunk) {
  var er = null;
  if (!util.isBuffer(chunk) &&
      !util.isString(chunk) &&
      !util.isNullOrUndefined(chunk) &&
      !state.objectMode) {
    er = new TypeError('Invalid non-string/buffer chunk');
  }
  return er;
}

function chunkInvalid(state, chunk) {
    var er = null;
    if (!util.isBuffer(chunk) && !util.isString(chunk) && !util.isNullOrUndefined(chunk) && !state.objectMode) {
      er = new TypeError('Invalid non-string/buffer chunk');
    }
    return er;
  }
  function onEofChunk(stream, state) {

function chunkInvalid(state, chunk) {
  var er = null;
  if (!util.isBuffer(chunk) &&
      !util.isString(chunk) &&
      !util.isNullOrUndefined(chunk) &&
      !state.objectMode) {
    er = new TypeError('Invalid non-string/buffer chunk');
  }
  return er;
}

function chunkInvalid(state, chunk) {
  var er = null;
  if (!util.isBuffer(chunk) &&
      !util.isString(chunk) &&
      !util.isNullOrUndefined(chunk) &&
      !state.objectMode) {
    er = new TypeError('Invalid non-string/buffer chunk');
  }
  return er;
}

function chunkInvalid(state, chunk) {
  var er = null;
  if (!util.isBuffer(chunk) &&
      !util.isString(chunk) &&
      !util.isNullOrUndefined(chunk) &&
      !state.objectMode) {
    er = new TypeError('Invalid non-string/buffer chunk');
  }
  return er;
}

Writable.prototype.write = function(chunk, encoding, cb) {
  var state = this._writableState;
  var ret = false;

  if (util.isFunction(encoding)) {
    cb = encoding;
    encoding = null;
  }

  if (util.isBuffer(chunk))
    encoding = 'buffer';
  else if (!encoding)
    encoding = state.defaultEncoding;

  if (!util.isFunction(cb))
    cb = function() {};

  if (state.ended)
    writeAfterEnd(this, state, cb);
  else if (validChunk(this, state, chunk, cb)) {
    state.pendingcb++;
    ret = writeOrBuffer(this, state, chunk, encoding, cb);
  }

  return ret;
};

Writable.prototype.write = function(chunk, encoding, cb) {
  var state = this._writableState;
  var ret = false;

  if (util.isFunction(encoding)) {
    cb = encoding;
    encoding = null;
  }

  if (util.isBuffer(chunk))
    encoding = 'buffer';
  else if (!encoding)
    encoding = state.defaultEncoding;

  if (!util.isFunction(cb))
    cb = function() {};

  if (state.ended)
    writeAfterEnd(this, state, cb);
  else if (validChunk(this, state, chunk, cb)) {
    state.pendingcb++;
    ret = writeOrBuffer(this, state, chunk, encoding, cb);
  }

  return ret;
};