5/7 Bleep Bleep: Both Codes work but why do we need this.lastTransactionAmount= 0


#1

https://www.codecademy.com/courses/close-the-super-makert/0/5?curriculum_id=506324b3a7dffd00020bf661#

I do not understand why code 1 is different from code 2. Please explain


//code 1
//
var cashRegister = {
    total:0,
    //Dont forget to add your property
    add: function(itemCost) {
        this.total +=  itemCost;
        lastTransactionAmount = itemCost;
    },
    scan: function(item,quantity) {
        switch (item) {
        case "eggs": this.add(0.98 * quantity); break;
        case "milk": this.add(1.23 * quantity); break;
        case "magazine": this.add(4.99 * quantity); break;
        case "chocolate": this.add(0.45 * quantity); break;
        }
        return true;
    },
    //Add the voidLastTransaction Method here
    voidLastTransaction: function(){
        this.total -= lastTransactionAmount;
        this.lastTransactionAmount = 0;
    }
    
};

cashRegister.scan('eggs',1);
cashRegister.scan('milk',1);
cashRegister.scan('magazine',1);
cashRegister.scan('chocolate',4);

//Void the last transaction and then add 3 instead
cashRegister.voidLastTransaction();
cashRegister.scan('chocolate',3);

//Show the total bill
console.log('Your bill is '+cashRegister.total);

//code 2//
var cashRegister = {
    total:0,
    //Dont forget to add your property
    add: function(itemCost) {
        this.total +=  itemCost;
        lastTransactionAmount = itemCost;
    },
    scan: function(item,quantity) {
        switch (item) {
        case "eggs": this.add(0.98 * quantity); break;
        case "milk": this.add(1.23 * quantity); break;
        case "magazine": this.add(4.99 * quantity); break;
        case "chocolate": this.add(0.45 * quantity); break;
        }
        return true;
    },
    //Add the voidLastTransaction Method here
    voidLastTransaction: function(){
        this.total -= lastTransactionAmount;
    }
    
};

cashRegister.scan('eggs',1);
cashRegister.scan('milk',1);
cashRegister.scan('magazine',1);
cashRegister.scan('chocolate',4);

//Void the last transaction and then add 3 instead
cashRegister.voidLastTransaction();
cashRegister.scan('chocolate',3);

//Show the total bill
console.log('Your bill is '+cashRegister.total);


#2

This variable is not scoped to the object execution context. Written like this it is a global variable with no ties to the object. The correct implementation is,

    total: 0,
    lastTransactionAmount: 0,
    add: function (itemCost) {
        this.total += itemCost;
        this.lastTransactionAmount = itemCost;
    },
    // and
    voidLastTransaction: function(){
        this.total -= this.lastTransactionAmount;
        this.lastTransactionAmount = 0;
    }

#3

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