Sort object by propriety not working

I followed the documentation about sort at https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/sort?v=example. It's not working. I even get a weirder issue: after hours, I discovered that if I change var comparator to function comparator(a,b), it works. Why is that??

test();

var comparator = (a, b) => {
    if (a.min !== b.min) {
        if (a.min < b.min) {
            return -1
        } else {
            return +1
        }
    } else {
        if (a.max !== b.max) {
            if (a.max < b.max) {
                return -1
            } else {
                return +1
            }
        } else {
            return 0;
        }
    }
}
function test() {
    for (var x = 0; x < 1; x++) {
        var tests = [{min: 18, max: 20},
            {min: 17, max: 20},
            {min: 10, max: 11}];
        console.log(tests.sort(comparator), 'test sorted');
    }

}

via TSR