Methods are Functions, Not Values

Beware of the difference between value properties and method properties!

• circle.radius is a value and evaluates to 2
• circle.area is a method and evaluates to function() {...}
• circle.area() is a method call and evaluates to 12.566370614359172

the class keyword

In 2015, JavaScript introduced the class keyword which is syntactic sugar on top of JavaScript's existing prototype system.

This new class syntax is much easier to understand than the previous system.

class Circle {
  circumference() {
    return Math.PI * this.radius * 2;
  }
  area() {
    return Math.PI * this.radius * this.radius;
  }
}

Use it like this:

let circle = new Circle();  // create a new Circle instance
circle.radius = 2;          // set its radius to 2
circle.area();              // call the area method, which
                            // returns 12.566370614359172 

• A Note On Spelling:
  • "Circle" with a capital C is the constructor
  • "circle" with a lowercase C is the object (instance)

MDN: classes

Stay classy, JavaScript

This is the first time we've seen classes in JavaScript

Classes are for making lots of objects with the same methods, but different data

A class defines a type of object.

An instance is an individual object of that type.

For example, there are many houses, but my house is yellow.

The Cookie Analogy:

• class ~= cookie cutter
• instance ~= cookie
• instance data ~= icing and sprinkles

Constructors and "new"

• A constructor is a function that's called when you use the new keyword
• It's the very first method that's ever called on that particular instance

What new does, in detail:

• make a new empty object
• set the object's class
• set this to point to the object
• call the constructor function
• set this back to whatever it was before
• return a reference to the object

Constructors are for Initialization

the principle of Complete Construction says that after the constructor executes, the object is in a valid state

in practice, this means "pass all initial values into the constructor"

A Better Circle:

class Circle {
  constructor(radius) {
    this.radius = radius;
  }
  circumference() {
    return Math.PI * this.radius * 2;
  }
  area() {
    return Math.PI * this.radius * this.radius;
  }
}

Use it like this:

let circle = new Circle(2);  // create a new Circle instance
                             // with radius 2
circle.area();              // call the area method, which
                            // returns 12.566370614359172 

Q: Why is this better?

A: because it preserves encapsulation -- the idea that an object should be responsible for setting its own properties

Constructors are for Validation

constructors are a great place to validate your values

class Circle {
    constructor(radius) {
        if (radius <= 0) {
            throw('radius must be a positive number')
        }
        this.radius = radius;
    }

• that if statement is known as a "guard clause"
  • it guards against bad values entering your algorithm
  • also known as "asserting your arguments" or "design by contract"
• validation is one of the most valuable features of object-oriented programming
  • it lets you write less code in other methods, confident that you don't have to check for bad data or boundary conditions

Factory Town

Sometimes one constructor just isn't enough.

When the constructor accepts different parameters from the ones that you have on hand, you could define a factory function like this:

function circleFromDiameter(diameter) {
    return new Circle(diameter / 2);
}

The above is called a "factory function" since it constructs objects for you, based on your specifications.

Factory Methods

For convenience and code organization, factory functions are often attached to the class -- not the instance -- of the objects they create.

The factory method works exactly the same way as the factory function, but

• the factory function is in the global namespace
• the factory method is in the class namespace so it's more clear that it is meant to create one of this class of objects

Static Factory Methods

To make a factory method in JavaScript, use the static keyword:

class Circle {
  static fromDiameter(diameter) {
    return new Circle(diameter / 2);
  }

  constructor(radius) {
    if (radius <= 0) {
        throw('radius must be a positive number')
    }
    this.radius = radius;
  }

  circumference() {
    return Math.PI * this.radius * 2;
  }

  area() {
    return Math.PI * this.radius * this.radius;
  }
}

and call it like this:

let circle = Circle.fromDiameter(4)

Note that (lowercase "c") circle.fromDiameter() does not work. Static methods are attached to classes, not instances.