[рони]

function fn(b) {
    for (var c = "", f = /(\S)(?=\S+?\1)/, a = 0; a < b.length; a++)
        for (var d = a + 1; d <= b.length; d++) {
            var e = b.substring(a, d);
            if (f.test(e)) break;
            else e.length > c.length && (c = e)
        }
    return c
};
var str = 'abcdeahopwunshslge';
alert(fn(str));