StringのArrayBufferへの変換と戻し(Blob() が無いと動かないので注意）

buffutil.js

var str2buff = function(str){
  var ab_ = new ArrayBuffer(new Blob([str]).size);
  var bytes_ = new Uint8Array(ab_);

  var n = str.length,
      idx = -1,
      i, c;

  for(i = 0; i < n; ++i){
    c = str.charCodeAt(i);
    if(c <= 0x7F){
      bytes_[++idx] = c;
    } else if(c <= 0x7FF){
      bytes_[++idx] = 0xC0 | (c >>> 6);
      bytes_[++idx] = 0x80 | (c & 0x3F);
    } else if(c <= 0xFFFF){
      bytes_[++idx] = 0xE0 | (c >>> 12);
      bytes_[++idx] = 0x80 | ((c >>> 6) & 0x3F);
      bytes_[++idx] = 0x80 | (c & 0x3F);
    } else {
      bytes_[++idx] = 0xF0 | (c >>> 18);
      bytes_[++idx] = 0x80 | ((c >>> 12) & 0x3F);
      bytes_[++idx] = 0x80 | ((c >>> 6) & 0x3F);
      bytes_[++idx] = 0x80 | (c & 0x3F);
    }
  }
  return bytes_;
}

var buff2str = function(buff){
  var size = buff.length;
  var i = 0, str = '', c, code;
  while(i < size){
    c = buff[i];
    if ( c < 128){
      str += String.fromCharCode(c);
      i++;
    } else if ((c ^ 0xc0) < 32){
      code = ((c ^ 0xc0) << 6) | (buff[i+1] & 63);
      str += String.fromCharCode(code);
      i += 2;
    } else {
      code = ((c & 15) << 12) | ((buff[i+1] & 63) << 6) |
        (buff[i+2] & 63);
      str += String.fromCharCode(code);
      i += 3;
    }
  }
  return str;
}

// test
var text = "abc";
var b = str2buff(text);
console.log(buff2str(b));

text = "こんにちは";
b = str2buff(text);
console.log(buff2str(b));