Getters, setters & #private fields

Guarded properties

Lesson 10 of 20 · 11 min · Module 3: Classes and objects

Some properties must always stay valid, and some should be calculated instead of stored. Getters and setters run code behind normal property syntax, and private fields keep the real data out of reach.

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By the end, you’ll be able to…

Compute properties with getters
Validate assignments with setters
Hide data with #private fields

The idea

get name() runs when the property is read; set name(value) runs when it is assigned. A field whose name starts with # is private: only code inside the class can read or change it.

class Person {
  #name;
  constructor(name) { this.name = name; }
  get name() { return this.#name; }
  set name(value) {
    if (!value.trim()) throw new Error("name required");
    this.#name = value.trim();
  }
}
console.log(new Person("  Ana ").name);

01Same syntax, more control

Callers write t.fahrenheit = 212 as with any property. Behind it, the setter converts and validates, so outside code never needs to know the details.

02Truly private

this.#celsius exists only inside the class body. Code outside cannot read it, change it, or even see it in Object.keys or JSON.stringify.

class Box {
  #secret = 42;
  reveal() { return this.#secret; }
}
const b = new Box();
console.log(Object.keys(b).length, b.reveal());

Output

0 42

03One source of truth

Only Celsius is stored. Fahrenheit is always derived from it, so the two values can never disagree.

Your turn

Write a class Temperature that stores degrees Celsius in a private field. celsius is a getter/setter that throws a RangeError below -273.15 (also in the constructor). fahrenheit is a getter/setter that converts with c * 9 / 5 + 32. Instances must have no visible own properties.

What the checks look for

Stores celsius and converts to fahrenheit
Setting fahrenheit updates celsius
Rejects temperatures below absolute zero
Keeps its data private

Try it in the editor →

Take this with you

Getters compute, setters guard, private fields protect.

Go a little further

Add a static method Temperature.fromFahrenheit(f) that creates an instance from a Fahrenheit value.