need a way to create an ArrayBuffer initialized with given byte array data. Perhaps a constructor that takes an array of bytes or a [Direct]ByteBuffer as an arg to use as the initial bytes for the array -

  ArrayBuffer(byte[] data);
  ArrayBuffer(java.nio.ByteBuffer data);

We could also add one that specifies offset and length for convenience -

  ArrayBuffer(byte[] data, int offset, int length);
  ArrayBuffer(java.nio.ByteBuffer data, int offset, int length);

V8 provides a similar API -

  static Local<ArrayBuffer> ArrayBuffer::New(void* data, size_t byte_length);