Definition of a flat string and example

I think there are a few things that could help you out - some of which Robin already mentioned:

• You can do a = new Uint8Array(10);a.set("Hello") to write direct into an ArrayBuffer - it'll be pretty fast. I'm not sure accepting a string is spec compliant but it works great on Espruino.
  
• new DataView(uint8array.buffer) is very fast - no copies or anything
  
• E.toString will create a flat string from whatever you give it - but the allocation of a flat string can be slow
  
• E.toUint8Array/E.toArrayBuffer may also help
  
• it should be reasonably efficient to append to a string as long as the string isn't too big
  
  

But yeah, doing something like:

var buf = new Uint8Array(200);
var idx = 0;
SerialX.on('data',function(d) {
  buf.set(d,idx);
  idx += d.length;
  // ...
});

works fine