Description

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Advanced · 12 min

Classes and encapsulation

Create instances that control their own state through methods.

Initialize an instance

A class groups construction and methods. new creates an instance and runs its constructor. Each instance can have its own values.

class Ticket {
  constructor(price) { this.price = price; }
  total(quantity) { return this.price * quantity; }
}
console.log(new Ticket(4).total(3));

Output

12

Protect internal state

A private field beginning with # can be accessed only within the declaring class. Expose methods that keep updates valid.

class Counter {
  #value = 0;
  increment() { this.#value += 1; }
  read() { return this.#value; }
}
const counter = new Counter();
counter.increment();
console.log(counter.read());

Output

1

Put it into practice

  1. Predict each result before running the example.
  2. Complete the coding task, then explain why your solution works.

Try it yourself

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Update private state

Make add increase the private quantity by the supplied amount. Display the quantity after adding 3.

Practice