Private properties

Defining private properties

Private properties in JavaScript are defined by prefixing the property name with a hash (#). This syntax ensures that the property cannot be accessed or modified directly from outside the class.

js

class SmartLight {
  #brightness // declare private property

  constructor(color, brightness) {
    this.color = color
    this.#brightness = brightness
    this.isOn = false
  }
}

const bathroomLight = new SmartLight('amber', 50)
console.log(bathroomLight)

output

SmartLight { isOn: false }

Notice that the brightness property is not logged to the console.

Private properties cannot be accessed directly

If we try and modify these properties directly, we will get an error:

js

bathroomLight.#brightness = 90

output

bathroomLight.#brightness = 90
              ^
SyntaxError: Private field '#color' must be declared in an enclosing class

Using private properties

Since private properties cannot be accessed directly from outside the class, we need to provide public methods to work with them.

class SmartLight {
  #brightness

  constructor(color, brightness) {
    this.color = color
    this.#brightness = brightness
  }

  getBrightness() {
    if (this.isOn) {
      return this.#brightness
    } else {
      return 0
    }
  }

  setBrightness(newBrightness) {
    if (newBrightness < 0 || newBrightness > 100) {
      throw new Error('Brightness must be between 0 and 100')
    }

    this.#brightness = newBrightness
  }
}