JavaScript Getters and Setters: get and set Properties

A getter looks like a property but runs a function when you read it. A setter runs a function when you assign to it.

script.js

const user = {
  firstName: "Maya",
  lastName: "Rao",

  get fullName() {
    return `${this.firstName} ${this.lastName}`;
  }
};

console.log(user.fullName); // → Maya Rao
user.firstName = "Meera";
console.log(user.fullName); // → Meera Rao

fullName is computed every time you read it, so it can never get out of date. There are no parentheses when you use it.

Setters

A setter receives the assigned value, which is good for validation.

script.js

const account = {
  _balance: 0,

  get balance() {
    return this._balance;
  },
  set balance(value) {
    if (value < 0) {
      console.log("Balance cannot be negative");
      return;
    }
    this._balance = value;
  }
};

account.balance = 100;
console.log(account.balance); // → 100
account.balance = -5;         // → Balance cannot be negative
console.log(account.balance); // → 100

The underscore in _balance is only a convention that says “treat this as internal”. The language does not enforce it (use private fields in classes for real privacy).

A getter without a setter is read-only

script.js

const circle = {
  radius: 5,
  get area() { return Math.round(Math.PI * this.radius ** 2); }
};

circle.area = 999;            // ignored in normal (sloppy) mode, an error in strict mode
console.log(circle.area);     // → 79

Getters in classes

You will use them most inside classes, see Getters and Setters in Classes.

When to use them

A value that is derived from other values (full name, total, area).
Validating or transforming data when it is assigned.

Do not use them for slow work, because they look like cheap property reads. Use a method (calculateTotal()) when the work is expensive or has side effects.