Objects and Classes

September 2018: This material is under very active development, and should all be considered the "brainstorming" phase of a rational lesson design process. We would appreciate your help: please email us, file an issue in our GitHub repository, or submit a pull request. (We would particularly appreciate descriptions of common errors and how to fix them.) Everyone whose work is incorporated will be acknowledged; please note that all contributors are required to abide by our Code of Conduct.


  • How can I use classes to keep code and data together?
  • What are the benefits of doing this?
  • How can I create an object from a class?
  • How can I initialize that object?
  • How can I create new classes from old?
  • How does JavaScript decide what to do when two classes define the same thing?
  • When should I create new classes and when should I combine existing ones?
  • How can old code use new code?
  • Making new code use old code is easy
  • How can we make old code use new code without rewriting?
  • Objects!

Doing It By Hand

  • An object is a set of key/value pairs
  • Values can be functions
  • So have data carry around functions that work on it
const square = {
  name: 'square',
  size: 5,
  area: (it) => { return it.size * it.size },
  perimeter: (it) => { return 4 * it.size }
  • Pass the object itself into the function
const a = square.area(square)
console.log(`area of square is ${a}`)
area of square is 25
  • This seems like a lot of work
  • But it allows us to handle many different kinds of things in the same way
const circle = {
  name: 'circle',
  radius: 3,
  area: (it) => { return Math.PI * it.radius * it.radius },
  perimeter: (it) => { return 2 * Math.PI * it.radius }

const rectangle = {
  name: 'rectangle',
  width: 2,
  height: 3,
  area: (it) => { return it.width * it.height },
  perimeter: (it) => { return 2 * (it.width + it.height) }

const everything = [square, circle, rectangle]
for (let thing of everything) {
  const a = thing.area(thing)
  const p = thing.perimeter(thing)
  console.log(`${thing.name}: area ${a} perimeter ${p}`)
square: area 25 perimeter 20
circle: area 28.274333882308138 perimeter 18.84955592153876
rectangle: area 6 perimeter 10
  • As long as we only use the value name and the functions area and perimeter we don’t need to know what kind of thing we’re actually working with
  • Polymorphism

  • But:
    • Building every object by hand is painful
    • Calling it.function(it) is clumsy


  • JavaScript solved these problems using prototypes
  • Most object-oriented languages use classes
    • These have been added to JavaScript ES6
    • We will use them instead of prototypes throughout
class Square {
  constructor (size) {
    this.name = 'square'
    this.size = size
  area () { return this.size * this.size }
  perimeter () { return 4 * this.size }

const sq = Square(3)
console.log(`sq name ${sq.name} and area ${sq.area()}`)
sq name square and area 9
  • new ClassName(...):
    • Creates a new blank object
    • Inserts a (hidden) reference to the class, so that the object can find its methods
    • Calls constructor to initialize the object’s state
    • Class names are written in CamelCase by convention
  • this is a pronoun that refers to a single specific object
  • Methods are defined with classic syntax rather than the fat arrows we have been using
  • Again, supports polymorphism
class Circle {
  constructor (radius) {
    this.name = 'circle'
    this.radius = radius
  area () { return Math.PI * this.radius * this.radius }
  perimeter () { return 2 * Math.PI * this.radius }

class Rectangle {
  constructor (width, height) {
    this.name = 'rectangle'
    this.width = width
    this.height = height
  area () { return this.width * this.height }
  perimeter () { return 2 * (this.width + this.height) }

const everything = [
  new Square(3.5),
  new Circle(2.5),
  new Rectangle(1.5, 0.5)
for (let thing of everything) {
  const a = thing.area(thing)
  const p = thing.perimeter(thing)
  console.log(`${thing.name}: area ${a} perimeter ${p}`)
square: area 12.25 perimeter 14
circle: area 19.634954084936208 perimeter 15.707963267948966
rectangle: area 0.75 perimeter 4


  • Build new classes from old by:
  • Start by defining a person
class Person {
  constructor (name) {
    this.name = name

  greeting (formal) {
    if (formal) {
      return `Hello, my name is ${this.name}`
    } else {
      return `Hi, I'm ${this.name}`
  • Then extend to create a scientist
    • Say that Scientist inherits from Person
class Scientist extends Person {
  constructor (name, area) {
    this.area = area

  greeting (formal) {
    return `${super.greeting(formal)}. Let me tell you about ${this.area}...`
  • Use super(...) in constructor to call up to parent’s constructor
    • Do not duplicate the steps it takes
  • An instance of Scientist will use Scientist.greeting, while instances of Person will use Person.greeting

FIXME-40: memory diagram

  • Result
const parent = new Person('Hakim')
console.log(`parent: ${parent.greeting(true)}`)

const child = new Scientist('Bhadra', 'microbiology')
console.log(`child: ${child.greeting(false)}`)
parent: Hello, my name is Hakim
child: Hi, I'm Bhadra. Let me tell you about microbiology...


  • Common use of object-oriented programming is to define a protocol
    • Actions that objects might take are defined in methods
    • Parent defines a method that invokes them at specific times or in a specific order
    • “You will all follow this procedure, but you may follow it in different ways”
  • How does a generic bird behave throughout the year?
class Bird {
  constructor (species) {
    this.species = species

  daily (season) {
    return [

  foraging (season) {
    return `${this.species} looks for food`

  mating (season) {
    let result = ''
    if (season === 'fall') {
      result = `${this.species} looks for a mate`
    return result

  nesting (season) {
    // do nothing
  • daily defines the bird’s overall behavior
  • foraging, mating, and nesting define default behaviors

  • How does a specific kind of bird behave?
class Penguin extends Bird {
  constructor () {
    this.hasEgg = false

  mating (season) {
    if (season === 'fall') {
      this.hasEgg = Math.random() < 0.5
    return super.mating(season)

  nesting (season) {
    let result = ''
    if (this.hasEgg && ((season === 'winter') || (season === 'spring'))) {
      result = `${this.species} is nesting`
      if (season === 'spring') {
        this.hasEgg = false
    return result
  • Has extra state (this.hasEgg)
    • Calls parent constructor before setting this up
  • Doesn’t override the default behavior for foraging
  • Extends the default behavior for mating
  • Replaces the default behavior for nesting

  • Result of some runs:
const bird = new Penguin()
const seasons = ['summer', 'fall', 'winter', 'spring']
for (let season of seasons) {
  console.log(`in ${season}: ${bird.daily(season)}`)
in summer: penguin looks for food,,
in fall: penguin looks for food,penguin looks for a mate,
in winter: penguin looks for food,,
in spring: penguin looks for food,,
  • Result of other runs:
in summer: penguin looks for food,,
in fall: penguin looks for food,penguin looks for a mate,
in winter: penguin looks for food,,penguin is nesting
in spring: penguin looks for food,,penguin is nesting
  • Different random numbers produce different behaviors
    • Makes testing hard
    • Look at how to address this in challenges
  • But main idea is how old code can use new code
    • Old code defines expectations as an interface and a protocol
    • New code implements that interface and respects that protocol



Define a class called Delay whose call method always returns the value given in the previous call:

const example = new Delay('a')
for (let value of ['b', 'c', 'd']) {
  console.log(value, '->', example.call(value))
b -> a
c -> b
d -> c

A class like Delay is sometimes called stateful, since it remembers its state from call to call.


Define a class called Filter whose call method returns null if its input matches one of the values given to its constructor, or the input as output otherwise:

const example = new Filter('a', 'e', 'i', 'o', 'u')
for (let value of ['a', 'b', 'c', 'd', 'e']) {
  console.log(value, '->', example.call(value))
a -> null
b -> b
c -> c
d -> d
e -> null

A class like Filter is sometimes called stateless, since it does not remember its state from call to call.


Define a class called Pipeline whose constructor takes one or more objects with a single-parameter call method, and whose own call method passes a value through each of them in turn. If any of the components’ call methods returns null, Pipeline stops immediately and returns null.

const example = new Pipeline(new Filter('a', 'e', 'i', 'o', 'u'), new Delay('a'))
for (let value of ['a' ,'b', 'c', 'd', 'e']) {
  console.log(value, '->', example.call(value)
a -> null
b -> a
c -> b
d -> c
e -> null

Active Expressions

Consider this class:

class Active {
  constructor (name, transform) {
    this.name = name
    this.transform = transform
    this.subscribers = []

  subscribe (someone) {

  update (input) {
    console.log(this.name, 'got', input)
    const output = this.transform(input)
    for (let s of this.subscribers) {

and this program that uses it:

const start = new Active('start', (x) => Math.min(x, 10))
const left = new Active('left', (x) => 2 * x)
const right = new Active('right', (x) => x + 1)
const final = new Active('final', (x) => x)

  1. Trace what happens when the last line of the program is called.
  2. Modify Active so that it calls transform if that function was provided, or a method Active.transform if a transformation function wasn’t provided.
  3. Create a new class Delay whose transform method always returns the previous value. (Its constructor will need to take an initial value as a parameter.)

This pattern is called observer/observable.

Key Points

  • Create classes to define combinations of data and behavior.
  • Use the class’s constructor to initialize objects.
  • this refers to the current object.
  • Use polymorphism to express common behavior patterns.
  • Extend existing classes to create new ones-sometimes.
  • Override methods to change or extend their behavior.
  • Creating extensible systems by defining interfaces and protocols.