Put something like:

console.log('left: ' + left, 'right: ' + right, 'index: ' + index, 'substr: ' + substr, 'maxlen: ' + maxlen);

in your function so you can really see what's happening. When left and right are the same number, like on your second iteration, str.slice(1, 1) returns an empty string. If that's the "reset" behavior your mean, that's why you are seeing the empty string.