array length

I'm afraid Espruino doesn't support setting the array length - however it's trivial to work around.

You can use either of:

• Array.slice : http://www.espruino.com/Reference#l_Arra­y_slice
  
• Array.splice : http://www.espruino.com/Reference#l_Arra­y_splice
  
• Array.pop : http://www.espruino.com/Reference#l_Arra­y_pop
  
• Array.shift : http://www.espruino.com/Reference#l_Arra­y_shift
  
  

Also, if you're after storing a whole lot of readings, maybe look at using Uint8Array/etc as they'll be vastly more efficient when storing large amounts of data.