Extract substring by utf-8 byte positions

I have a string and start and length with which to extract a substring. Both positions (start and length) are based on the byte offsets in the original UTF8 string.

However, there is a problem:

The start and length are in bytes, so I cannot use "substring". The UTF8 string contains several multi-byte characters. Is there a hyper-efficient way of doing this? (I don't need to decode the bytes...)

Example： var orig = '你好吗？'

The s,e might be 3,3 to extract the second character (好). I'm looking for

var result = orig.substringBytes(3,3);

Help!

Update #1 In C/C++ I would just cast it to a byte array, but not sure if there is an equivalent in javascript. BTW, yes we could parse it into a byte array and parse it back to a string, but it seems that there should be a quick way to cut it at the right place. Imagine that 'orig' is 1000000 characters, and s = 6 bytes and l = 3 bytes.

Update #2 Thanks to zerkms helpful re-direction, I ended up with the following, which does NOT work right - works right for multibyte but messed up for single byte.

function substrBytes(str, start, length)
{
    var ch, startIx = 0, endIx = 0, re = '';
    for (var i = 0; 0 < str.length; i++)
    {
        startIx = endIx++;

        ch = str.charCodeAt(i);
        do {
            ch = ch >> 8;   // a better way may exist to measure ch len
            endIx++;
        }
        while (ch);

        if (endIx > start + length)
        {
            return re;
        }
        else if (startIx >= start)
        {
            re += str[i];
        }
    }
}

Update #3 I don't think shifting the char code really works. I'm reading two bytes when the correct answer is three... somehow I always forget this. The codepoint is the same for UTF8 and UTF16, but the number of bytes taken up on encoding depends o