Javascript algorithm for rounding down an integer to the nearest power of 2 using bitwise operators

roundDownPow2.js

//modified from http://graphics.stanford.edu/~seander/bithacks.html#RoundUpPowerOf2
//input of 1 returns 1

function nextLowestPow2(d) {
    d|=d>>1;
    d|=d>>2;
    d|=d>>4;
    d|=d>>8;
    d|=d>>16;
    return (d + 1) / 2;
}

nextLowestPow2(4); //returns 4
nextLowestPow2(7); //returns 4
nextLowestPow2(83); //returns 64