function tipCalculator(bill) {

    /**optional */
    if (bill < 0 || isNaN(bill)) {
        console.log('oops')
        return NaN;
    };
    /*** */

    let tip;
    switch (true) {

        case (bill < 50):
            tip = bill * 0.2;
            break;

        case (bill >= 50 && bill <= 200):
            tip = bill * 0.15;
            break;

        default:
            tip = bill * 0.1;
            break;

    }
    return tip;
}

var bills = [124, 48, 268];
var AmountDue = bills.map(bill => (bill + tipCalculator(bill)).toFixed(2));
console.log(AmountDue)