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function payBill(){
 
    // valid promo codes for current month to get discount on total payable amount
    var validCodes = ['FIRST50', 'OFF10', 'APP15', 'WALLET'];
 
    function checkCode(promoCode){
 
        if( validCodes.indexOf(promoCode) !== -1 )
        {
            console.log('Hurray!! You got a discount.')
        }
        else{
            console.log('You need to pay full amount.');
        }    
    }
 
    return checkCode;
}
 
var checkCodeFn = payBill();
 
checkCodeFn('DISCOUNT');    // You need to pay full amount.
checkCodeFn('FIRST50');     // Hurray!! You got a discount.