LedMatrix for 8*8 WS2812

You can do:

JsVar* v = jsvArrayBufferGet(arrayBuffer, 30);
jsvArrayBufferSet(arrayBuffer, 60, v);
jsvUnLock(v);

But that's not as fast. The ideal thing is to read or write data sequentially:

JsvArrayBufferIterator it;
jsvArrayBufferIteratorNew(&it, start_index);
while (pixels--) {
  jsvArrayBufferIteratorSetByteValue(&it, value);
  jsvArrayBufferIteratorNext(&it);
}
jsvArrayBufferIteratorFree(&it);

Your other option is to use 'flat strings' - if you can use them then when you use JSV_GET_AS_CHAR_ARRAY you'll get a pointer to the underlying data, so there is no need to copy back.

The ability to scroll with the Graphics library would actually be really useful, although that's a bit tricker to deal with as you need to cope with bit-based graphics where the pixels are in strange formats...

No problem if you're doing this for fun yourself - but I don't see I'll be able to include this code in Espruino itself - there's just too little flash memory available for me to be be able to use it on something that's very single-purpose.