A JavaScript UTF-8 decoding function for ArrayBuffers. Credit for most of the heavy lifting goes to "bob" http://ciaranj.blogspot.com/2007/11/utf8-characters-encoding-in-javascript.html

gistfile1.js

function decodeUtf8(arrayBuffer) {
  var result = "";
  var i = 0;
  var c = 0;
  var c1 = 0;
  var c2 = 0;

  var data = new Uint8Array(arrayBuffer);

  // If we have a BOM skip it
  if (data.length >= 3 && data[0] === 0xef && data[1] === 0xbb && data[2] === 0xbf) {
    i = 3;
  }

  while (i < data.length) {
    c = data[i];

    if (c < 128) {
      result += String.fromCharCode(c);
      i++;
    } else if (c > 191 && c < 224) {
      if( i+1 >= data.length ) {
        throw "UTF-8 Decode failed. Two byte character was truncated.";
      }
      c2 = data[i+1];
      result += String.fromCharCode( ((c&31)<<6) | (c2&63) );
      i += 2;
    } else {
      if (i+2 >= data.length) {
        throw "UTF-8 Decode failed. Multi byte character was truncated.";
      }
      c2 = data[i+1];
      c3 = data[i+2];
      result += String.fromCharCode( ((c&15)<<12) | ((c2&63)<<6) | (c3&63) );
      i += 3;
    }
  }
  return result;
}

Trimmed version with full Unicode support https://gist.github.com/pascaldekloe/62546103a1576803dade9269ccf76330