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Classes and instances ​

In programming, a class is like a blueprint for creating objects. These objects are known as instances of the class. Using a class for object creation means objects are created the same way every time, which helps us adhere to DRY (don't repeat yourself) principles.

Defining a class ​

We create a class using the class keyword.

js
class SmartLight {
  // class implementation goes here
}

In javascript, it is conventional capitalise the first letter of class names.

Creating instances ​

To create a smart light instance from this blueprint, we use the new keyword like this:

js
class SmartLight {
  // class implementation goes here
}

const kitchenLight = new SmartLight()
const bedroomLight = new SmartLight()

console.log(kitchenLight)
console.log(bedroomLight)
console
SmartLight {}
SmartLight {}

The output shows us that we have two instances of SmartLight. But they don't have any properties yet. They are empty objects.

Constructor function ​

A constructor is a special function inside a class that sets up new objects. It's where we initialise properties.

js
class SmartLight {
  constructor(color, brightness) {
    this.color = color
    this.brightness = brightness
  }
}

const kitchenLight = new SmartLight('warm white', 75)
const bedroomLight = new SmartLight('cool blue', 50)

console.log(kitchenLight)
console.log(bedroomLight)
console
SmartLight { color: 'warm white', brightness: 75 }
SmartLight { color: 'cool blue', brightness: 50 }

Think of this as an unfinished smart light on the production line - we're adding things to it as we go.

Additional properties ​

The constructor can add properties that are not passed in as arguments.

js
class SmartLight {
  constructor(color, brightness) {
    this.color = color
    this.brightness = brightness
    this.isOn = false // lights are off by default
  }
}

const kitchenLight = new SmartLight('warm white', 75)

console.log(kitchenLight)
console
SmartLight {
  color: 'warm white',
  brightness: 75,
  isOn: false
}