JavaScript Class Getter and Setter: get and set Keywords

In a class, get and set define accessor properties: they look like fields from the outside, but run code.

script.js

class Rectangle {
  constructor(width, height) {
    this.width = width;
    this.height = height;
  }

  get area() {
    return this.width * this.height;
  }
}

const rect = new Rectangle(4, 5);
console.log(rect.area);   // → 20    (no parentheses)
rect.width = 10;
console.log(rect.area);   // → 50    (always up to date)

area is computed each time it is read, so it can never be out of date, and you do not store it.

Setters validate

script.js

class Temperature {
  #celsius = 0;

  get celsius() {
    return this.#celsius;
  }

  set celsius(value) {
    if (typeof value !== "number" || Number.isNaN(value)) {
      throw new TypeError("Temperature must be a number");
    }
    if (value < -273.15) {
      throw new RangeError("Below absolute zero");
    }
    this.#celsius = value;
  }

  get fahrenheit() {
    return this.#celsius * 9 / 5 + 32;
  }

  set fahrenheit(value) {
    this.celsius = (value - 32) * 5 / 9;
  }
}

const t = new Temperature();
t.celsius = 25;
console.log(t.fahrenheit);   // → 77

t.fahrenheit = 212;
console.log(t.celsius);      // → 100

try {
  t.celsius = -500;
} catch (error) {
  console.log(error.name);   // → RangeError
}

Together with a private field, the setter guarantees that the stored value is always valid.

Read-only properties

A getter without a setter is read-only. Assigning to it is silently ignored in sloppy mode, and throws a TypeError in strict mode (inside modules, classes and "use strict" code).

script.js

class Circle {
  constructor(radius) { this.radius = radius; }
  get diameter() { return this.radius * 2; }
}

const c = new Circle(3);

function tryToChange() {
  "use strict";              // code outside a class is only strict if you ask for it
  c.diameter = 10;
}

try {
  tryToChange();
} catch (error) {
  console.log(error.name);   // → TypeError
}
console.log(c.diameter);     // → 6

Getters in the console

They are on the prototype, so they are not shown as own properties when you log the object, and JSON.stringify skips them.

Use them for

Values derived from other fields (full name, total, area).
Validation when a field is set.
Keeping the public interface stable while the internal storage changes.

Do not hide slow or surprising work behind a getter. If it takes time, call it a method (calculateTotal()).

Static getters

static get version() { ... } also exists.

info

Same idea as in objects

This is the class version of object getters and setters.