Javascript - Swap adjacent letters

gistfile1.js

letterSwap = function(str, swap) {
    ot = str.split('');
    for( i in str) {
        if(i%swap===0) {
            ot[i] = str[parseInt(i)+swap-1];
            ot[parseInt(i)+swap-1] = str[i];
        }
    }
    return ot.join('');
}

console.log(letterSwap('ABCDEFGHIJKLMNOPQRSTUVWXYZ', 3));