How to convert byte string into Number?

I have a string encoded in Little Endian format (least-significant-byte first), and I want to decode开发者_运维问答 it into a Number. For the encoder, the first bit of the last byte reflects whether the number is positive or negitive. Does anyone know how to do this?

Here's my decoder:

decode:function(str,type){
        var num=0;
        var len=size(type)-(type.signed?1:0);
        var totalBits=0;
        for(var i=0;i<len;i++){
            num+=(str.charCodeAt(i)<<totalBits));
            totalBits+=8;
        }
        if(type.signed){
            var b=str.charCodeAt(size(type)-1);
            var neg=(b>=128);
            if(neg){
                b-=128;
            }
            num+=b;
            num*=(neg?-1:1);
        }
        return num;
    }

Thanks.

---

It's easier if you read the string from right to left. You could do something like this:

/* decode. assume size(str) > 0 */
function (str,type) {
  var len = size(str);
  var num = str.charCodeAt(len - 1);
  if (type.signed && (num >= 128)) {
    num = num - 256;
  }
  for (var i = (len - 2); i >= 0; i--) {
    num = 256 * num + str.charCodeAt(i);
  }
  return num;
}