How to use the buffer.Buffer.allocUnsafeSlow function in buffer

if (err)
    return context.close(err);

  const size = context.size = isFileType(stats, S_IFREG) ? stats[8] : 0;

  if (size > kMaxLength) {
    err = new ERR_FS_FILE_TOO_LARGE(size);
    return context.close(err);
  }

  try {
    if (size === 0) {
      context.buffers = [];
    } else {
      context.buffer = Buffer.allocUnsafeSlow(size);
    }
  } catch (err) {
    return context.close(err);
  }
  context.read();
}

ReadFileContext.prototype.read = function() {
  var buffer;
  var offset;
  var length;

  if (this.size === 0) {
    buffer = this.buffer = Buffer.allocUnsafeSlow(kReadFileBufferLength);
    offset = 0;
    length = kReadFileBufferLength;
  } else {
    buffer = this.buffer;
    offset = this.pos;
    length = this.size - this.pos;
  }

  var req = new FSReqWrap();
  req.oncomplete = readFileAfterRead;
  req.context = this;

  binding.read(this.fd, buffer, offset, length, -1, req);
};

ReadFileContext.prototype.read = function() {
  var buffer;
  var offset;
  var length;

  if (this.size === 0) {
    buffer = this.buffer = Buffer.allocUnsafeSlow(kReadFileBufferLength);
    offset = 0;
    length = kReadFileBufferLength;
  } else {
    buffer = this.buffer;
    offset = this.pos;
    length = this.size - this.pos;
  }

  var req = new FSReqWrap();
  req.oncomplete = readFileAfterRead;
  req.context = this;

  binding.read(this.fd, buffer, offset, length, -1, req);
};

var size = context.size = st.isFile() ? st.size : 0;

  if (size === 0) {
    context.buffers = [];
    context.read();
    return;
  }

  if (size > kMaxLength) {
    err = new RangeError('File size is greater than possible Buffer: ' +
                         `0x${kMaxLength.toString(16)} bytes`);
    return context.close(err);
  }

  context.buffer = Buffer.allocUnsafeSlow(size);
  context.read();
}

var size = context.size = st.isFile() ? st.size : 0;

  if (size === 0) {
    context.buffers = [];
    context.read();
    return;
  }

  if (size > kMaxLength) {
    err = new RangeError('File size is greater than possible Buffer: ' +
                         `0x${kMaxLength.toString(16)} bytes`);
    return context.close(err);
  }

  context.buffer = Buffer.allocUnsafeSlow(size);
  context.read();
}

var size = context.size = st.isFile() ? st.size : 0;

  if (size === 0) {
    context.buffers = [];
    context.read();
    return;
  }

  if (size > kMaxLength) {
    err = new RangeError('File size is greater than possible Buffer: ' +
                         `0x${kMaxLength.toString(16)} bytes`);
    return context.close(err);
  }

  context.buffer = Buffer.allocUnsafeSlow(size);
  context.read();
}

var size = context.size = st.isFile() ? st.size : 0;

  if (size === 0) {
    context.buffers = [];
    context.read();
    return;
  }

  if (size > kMaxLength) {
    err = new RangeError('File size is greater than possible Buffer: ' +
                         `0x${kMaxLength.toString(16)} bytes`);
    return context.close(err);
  }

  context.buffer = Buffer.allocUnsafeSlow(size);
  context.read();
}

var size = context.size = st.isFile() ? st.size : 0;

  if (size === 0) {
    context.buffers = [];
    context.read();
    return;
  }

  if (size > kMaxLength) {
    err = new RangeError('File size is greater than possible Buffer: ' +
                         `0x${kMaxLength.toString(16)} bytes`);
    return context.close(err);
  }

  context.buffer = Buffer.allocUnsafeSlow(size);
  context.read();
}

ReadFileContext.prototype.read = function() {
  var buffer;
  var offset;
  var length;

  if (this.size === 0) {
    buffer = this.buffer = Buffer.allocUnsafeSlow(kReadFileBufferLength);
    offset = 0;
    length = kReadFileBufferLength;
  } else {
    buffer = this.buffer;
    offset = this.pos;
    length = this.size - this.pos;
  }

  var req = new FSReqWrap();
  req.oncomplete = readFileAfterRead;
  req.context = this;

  binding.read(this.fd, buffer, offset, length, -1, req);
};

var size = context.size = st.isFile() ? st.size : 0;

  if (size === 0) {
    context.buffers = [];
    context.read();
    return;
  }

  if (size > kMaxLength) {
    err = new RangeError('File size is greater than possible Buffer: ' +
                         `0x${kMaxLength.toString(16)} bytes`);
    return context.close(err);
  }

  context.buffer = Buffer.allocUnsafeSlow(size);
  context.read();
}