LESSON 409 OF 410 · PROJECTS: OBJECT-ORIENTED

Vending Machine in JavaScript in JavaScript

Practice

Build a VendingMachine class for a machine that sells one kind of item for a fixed price. It keeps the money inserted so far (the credit), sells an item when the credit is enough, and gives the remaining credit back on request. A new object is made with new VendingMachine().

Rules:

State. A new machine has credit 0 and price 150 (every item costs 150). Declare the class fields credit = 0; and price = 150;.
insert(amount) adds amount to the credit. Nothing is printed or returned.
select() tries to buy one item and returns a string:
- If the credit is less than the price, return "Need N more", where N is the missing amount (price - credit) as a whole number, for example "Need 100 more". The credit does not change.
- Otherwise (the credit is equal to or greater than the price, so exact payment works), subtract the price from the credit and return "Dispensing!". Any extra money stays in the credit.
refund() returns the whole credit and sets it back to 0, so asking again right away returns 0.

You start with this driver below your code:

let m = new VendingMachine();
m.insert(50);
console.log(m.select());
m.insert(150);
console.log(m.select());
console.log("Refunded: " + m.refund());
m.insert(150);
console.log(m.select());
console.log("Refunded: " + m.refund());
m.insert(100);
console.log(m.select());
console.log("Refunded: " + m.refund());

Walk through it: 50 is not enough, so you see Need 100 more. After 150 more the credit is 200, so the machine dispenses and keeps 50 (200 - 150), and the refund is 50. Then 150 is exact payment: it dispenses and the refund is 0. Finally 100 is short by 50, the credit stays 100, and the refund is 100.

Expected output:

Need 100 more
Dispensing!
Refunded: 50
Dispensing!
Refunded: 0
Need 50 more
Refunded: 100

Join the missing amount into the text with +: "Need " + (this.price - this.credit) + " more". Do the subtraction only when the item is dispensed, not when the credit is too low. If you forget it, the leftover credit is wrong and the refund is too big.

In the refund method, save the credit first, then reset it to 0, then return what you saved. Return the saved constant, not this.credit.

class VendingMachine {
    credit = 0;
    price = 150;

    insert(amount) {
        this.credit += amount;
    }

    select() {
        if (this.credit < this.price) return "Need " + (this.price - this.credit) + " more";
        this.credit -= this.price;
        return "Dispensing!";
    }

    refund() {
        const r = this.credit;
        this.credit = 0;
        return r;
    }
}

STEP 1 OF 13

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