问单字节javascript上的位明智计数器

EN

0000111100000000 ->15
0000111000000000 ->14
0000110100000000 ->13
...
0000000000000000 ->0

cellBinary = iterate(cellBinary, 16, 4);
function iterate(cellBinary, start, length)
    {
    let number = extractBits(cellBinary, start, length);
    if(number < 15)
        {
        number++;
    }
    what = eraseIterator(what);
    what = what|number;
    return what;
}
function extractBits(what, start, length)
    {
    return ((1 << start ) -1) & (what >> (length - 1));
}
function eraseIterator(what)
    {
    what&16^1; //also iffy if this will work as intended.
    what&32^1; //this is supposed to set 16-128 to 0.
    what&64^1;
    what&128^1; 
    return what;
}