Classes

Learning objectives#

• Define classes with constructors and methods
• Use getters, setters and private #fields
• Add static properties
• Extend classes with extends and super

Explanation#

class Product { constructor(name, price) { … } } defines a blueprint; new Product('Book', 500) creates an object from it.

Methods are shared by every instance. get and set let properties run code when read or written.

Fields starting with # are private — only code inside the class can access them.

static members belong to the class itself (Product.count), not to instances. extends creates a subclass; super calls the parent.

Code example: Products and digital products#

1class Product {
2  static count = 0;
3  #discount = 0;
4

5  constructor(name, price) {
6    this.name = name;
7    this.price = price;
8    Product.count++;
9  }
10  set discount(pct) {
11    if (pct < 0 || pct > 90) throw new RangeError('Discount must be 0–90%');
12    this.#discount = pct;
13  }
14  get finalPrice() {
15    return this.price * (1 - this.#discount / 100);
16  }
17  toString() {
18    return `${this.name}: ₹${this.finalPrice}`;
19  }
20}
21

22class DigitalProduct extends Product {
23  constructor(name, price, sizeMb) {
24    super(name, price);
25    this.sizeMb = sizeMb;
26  }
27  toString() {
28    return `${super.toString()} (${this.sizeMb} MB download)`;
29  }
30}
31

32const book = new Product('Book', 500);
33book.discount = 10;
34const course = new DigitalProduct('Video course', 1200, 850);
35

36console.log(String(book));
37console.log(String(course));
38console.log(Product.count, course instanceof Product);

Book: ₹450
Video course: ₹1200 (850 MB download)
2 true

Step-by-step explanation#

1. The constructor sets up each product and increases the static counter.
2. The discount setter validates the value before saving it in the private #discount field.
3. finalPrice is a getter, so it is read like a property: book.finalPrice.
4. DigitalProduct extends Product, calls super(...) and reuses super.toString().
5. Two products were created, and a DigitalProduct is also a Product.

Real-world use case#

Common mistakes#

Best practices#

• Keep classes small and focused
• Use private #fields for internal state
• Validate in setters or constructors
• Prefer composition over deep inheritance

Remember this#

Practice exercise#

1class BankAccount {
2  #balance = 0;
3  deposit(a) { this.#balance += a; }
4  withdraw(a) { if (a > this.#balance) throw new Error('Insufficient funds'); this.#balance -= a; }
5  get balance() { return this.#balance; }
6}
7const acc = new BankAccount();
8acc.deposit(500); acc.withdraw(200);
9console.log(acc.balance); // 300

Mini quiz#

Want more? Practise all JavaScript MCQs · JavaScript interview questions

Related topics#

My notes#

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