object oriented javascript: extending classes

2020-04-07

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~5 min read

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994 words

I’ve written in the past about OO in Javascript, but recently I was working on learning more about heaps and as I pulled together my own implementation of a binary heap, I noticed a gap in my understanding specifically as it relates to extending classes in Javascript.

In my previous OO post in Javascript, I wrote about the different styles of writing “classes” (which are really just functions that return objects - though their differences, particularly as they relate to memory management are important to keep in mind):

The focus of that article, however was about designing classes, not about extending or subclassing. Let’s tackle that topic now.

Pseudoclassical SubClassing

First up: extending a “class” with Javascript’s pseudoclassical pattern. As a basis for example, I’ll revisit the Car class from my previous post:

function Car(make, model, year) {
  this.make = make
  this.model = model
  this.year = year
}
Car.prototype.wheels = 4
Car.prototype.trunk = true
Car.prototype.doors = 4
Car.prototype.greet = function() {
  return `Hi, I am a ${this.make} ${this.model}`
}

How might it look if I wanted a Truck class? A truck is similar to a car, but it’s special. It has two-doors by default (in my mind) and it’s towing capacity is a key characteristic.

I also think its personality would be different. Let’s make a greeting that’s moar.

function Truck(make, model, year, towingCapacity, doors) {
  Car.call(this, make, model, year)
  this.towingCapacity = towingCapacity
  this.doors = doors || 2
}

Truck.prototype = Object.create(Car.prototype)
Truck.prototype.greet = function() {
  return `I want moaaaaar! I'm a ${this.make} ${this.model} from ${this.year}`
}

Nota Bene

  1. When extending the Truck class’s prototype, it’s critical to use Object.create(). Without it, the Truck prototype would point to the same object in memory as the Car. As a result, any change made to the Truck prototype would then be reflected on the car.
function Car(/*...*/) {
  /*...*/
}
Car.prototype.greet = function() {
  return `Hi, I am a ${this.make} ${this.model}`
}
/*...*/
function Truck(/*...*/) {
  /*...*/
}
Truck.prototype = Car.prototype
Truck.prototype.greet = function() {
  return `I want moaaaaar! I'm a ${this.make} ${this.model} from ${this.year}`
}
const myCar = new Car(/*...*/)
console.log(myCar.greet()) // I want moaaaaar! I'm a ...
  1. Because Object.create creates a brand new object in memory, at that point, the prototypes diverge. This means that a “subclass” may not have access to all of the same methods as its parent if that method was added later.
function Car(/*...*/) {
  /*...*/
}
/*...*/
function Truck(/*...*/) {
  /*...*/
}
Truck.prototype = Object.create(Car.prototype)
Car.prototype.eject = function() {
  return `Preparing eject sequence. 5...4...3...2..1...`
}
const myTruck = new Truck(/*...*/)
myTruck.eject() // TypeError: myTruck.eject is not a function

Class Sugar

Now that we’ve extended a class with the pseudoclassical approach, let’s look at how we might do the same but with the syntactic sugar offered by the class keyword. The car, again, might look like:

class Car {
  constructor(make, model, year) {
    this.make = make
    this.model = model
    this.year = year
  }
  wheels = 4
  trunk = true
  doors = 4
  greet = function() {
    return `Hi, I am a ${this.make} ${this.model}`
  }
}

The first thing to notice is that the entire class is defined within one object (though this isn’t strictly necessary). “Extending” the class is now much clearer because of the extends keyword.

class Truck extends Car {
  constructor(make, model, year, towingCapacity, doors) {
    super(make, model, year)
    this.towingCapacity = towingCapacity
    this.doors = doors || 2
  }
  greet = function() {
    return `I want moaaaaar! I'm a ${this.make} ${this.model} from ${this.year}`
  }
}

As a reminder for what’s happening here:

  • The constructor function replaces the function signature.
  • The invocation of super replaces the Car.call(this,...) but does the same thing. Because Truck extends Car, when we call super, we’re instantiated a Car class. We have the opportunity to override the parent class values though. While this example doesn’t, imagine a constructor that had another property this.year = year - 2. Now, instead of using the year in our class returned by the Car class, our year would be two less.

The biggest benefit here is readability. The class keyword helps to colocate the class logic in a more intuitive (to me) way.

This self-containment mitigates the risk of sprinkling logic around in different places that can result in diverging prototypes. More to the point, however, the class keyword uses the same prototypal chain. The result is that we can extend a parent class whenever and our subclasses will still have access to those new methods.

Take for example, defining a new method on our Car class after the Truck has been defined.

For example:

class Truck extends Car {
  /*...*/
}
const myTruck = new Truck('tesla', 'cyber truck', 2019, 1000)
Car.prototype.start = function() {
  return `START YOUR ENGINES!`
}
console.log(myTruck.start()) // START YOUR ENGINES

Prototypal inheritance rules still apply however. If a more localized version of the method start is present, it will be invoked (and this is true if the start function is defined in the class or later - as was the case with the Car’s start method):

class Truck extends Car {
  /*...*/
  start = function() {
    return `VROOM! Let's Go!`
  }
}
const myTruck = new Truck('tesla', 'cyber truck', 2019, 1000)
Car.prototype.start = function() {
  return `START YOUR ENGINES!`
}
console.log(myTruck.start()) // VROOM! Let's Go!

Conclusion

Every time I dip back into object oriented programming, I learn something. This is part and parcel of my favorite part about programming: there’s no one way to solve a problem, though different approaches lend themselves to problems better than others.

Understanding how to take advantage of OO in Javascript is important to me so that when I encounter problems that are well suited for it, I’m ready.

Hopefully this was helpful for you!


Hi there and thanks for reading! My name's Stephen. I live in Chicago with my wife, Kate, and dog, Finn. Want more? See about and get in touch!