Classes in CoffeeScript

Core points

• CoffeeScript implements traditional class systems, although JavaScript itself does not. This makes it easier for beginners to understand while retaining the flexibility of prototypes for experienced programmers.
• CoffeeScript class supports inheritance, allowing the creation of subclasses that automatically inherit the properties and methods of their parent class. Subclasses can also override parent class functions, as shown in the "worry" and "profit" functions in the "Senator" and "Student" subclasses.
• Although CoffeeScript is convenient and syntaxy-concise, it still allows prototype systems that implement JavaScript, including using "::" as a shortcut to "prototype", and using the "extends" and "super" keywords for constructors.

JavaScript does not have traditional class systems, but uses prototypes. The prototype is very flexible and powerful, but it is confusing for beginners. Therefore, CoffeeScript creates a traditional class system. But…how did it do it? The slogan of CoffeeScript is "It's just JavaScript", and JavaScript obviously lacks the traditional class system. In this article, we will cover the basics of creating a CoffeeScript class. In the next post, we will dig into generated JavaScript (relatively advanced) to see the magic behind it.

Parent class

We will mainly explain it through examples, because for those who have read my introduction to CoffeeScript, it should be easy to understand what is going on.

We declare a class called Bourgeoisie. The function declaration of the class is as follows:

class Bourgeoisie
  constructor: (@age, @privilegeConstant) ->

  worry: ->
    console.log("我的股票下跌了 1%！")

  profit: (hardWork, luck) ->
    return (@age - 23) * hardWork * (luck + @privilegeConstant)

elite = new Bourgeoisie(29, 397)
elite.worry() # "我的股票下跌了 1%！"
elite.profit(20, 50) # 53640

The constructor is named constructor. It accepts two parameters, age and privilegeConstant, and automatically assigns them as instance variables (@ is a replacement for this in CoffeeScript, and the variables are automatically assigned to the instance when used in the parameters of the constructor). When you create a new Bourgeoisie, the constructor is called automatically, just like at the bottom of the code example. We have two other functions. The first function worry does not accept any parameters. The second function profit accepts two arguments and returns a number.

functionName: (arguments) ->
  code

Inheritance class

Now, we want a class that inherits from Bourgeoisie. We name it Senator.

This class

class Senator extends Bourgeoisie
  worry: ->
    console.log("民调下跌了 1%！")

senator = new Senator(45, 992)
senator.worry() # "民调下跌了 1%！"
senator.profit(6, 10) # 132264

extends

Bourgeoisie, which means it has all the features of the parent class. The constructor is exactly the same as the profit function , the only difference is that when constructing an instance, you are calling Senator instead of Bourgeoisie. On the other hand, the worry function is different. Senator is more worried about polls than stocks, so his worry covers the worry of the parent class. This override can be seen again in the Student class shown below.

class Bourgeoisie
  constructor: (@age, @privilegeConstant) ->

  worry: ->
    console.log("我的股票下跌了 1%！")

  profit: (hardWork, luck) ->
    return (@age - 23) * hardWork * (luck + @privilegeConstant)

elite = new Bourgeoisie(29, 397)
elite.worry() # "我的股票下跌了 1%！"
elite.profit(20, 50) # 53640

Student's worry overwrites the parent class's worry (more dramatic than the Senator's way), and their profit is also overwritten. However, overwriting now depends on the profit function of the parent class. It takes this function and minus tuition fees. This is a difficult time for students! But what you really should learn from it is the super keyword, which calls the function version of the parent class.

But, I like prototypes

Very good! CoffeeScript is for you, but it still provides you with powerful features. Let's use it! Here is a brief example from our last time:

functionName: (arguments) ->
  code

This should look familiar, as 90% of the changes are to replace some functions with pointed arrows. The prototype inheritance system remains the same because remember that CoffeeScript is just JavaScript. The syntax is more concise, and if you want to prototype in the Brendan Eich style we used last time, you don't need to learn more. This is not to say that CoffeeScript does not apply some shortcuts. You can use :: instead of prototype. However, in Brendan Eich style prototypes, we only need to use it once in the object(o) method. We also have access to extends and super keywords, but these keywords are only used in the constructor - we hide it in the object(o) method again.

Conclusion

The classic inheritance system in CoffeeScript provides convenience and comfort for ordinary developers. In addition, CoffeeScript's cleaner syntax makes it easier to implement a real prototype system. In short, there are only benefits but no harm.

FAQs about CoffeeScript class

What is the meaning of a class in CoffeeScript?

CoffeeScript classes are a basic concept that helps organize and build code. They provide a way to create objects with specific properties and methods. Classes make creating complex data structures easier and provide a way to encapsulate data and functionality in one place. They also support inheritance, allowing you to create a new class that inherits from an existing class. This can greatly reduce code duplication and improve code maintainability.

How to define a class in CoffeeScript?

It is simple to define a class in CoffeeScript. You use the class keyword followed by the class name. For example, class MyClass. You can then use the @ symbol to refer to an instance of the class to define properties and methods in the class. For example, @myProperty will define the properties of the class, while @myMethod: -> will define the methods.

How does inheritance in CoffeeScript class work?

Inheritance in CoffeeScript is implemented using the extends keyword. When you define a class, you can specify that it extends another class. This means that the new class will inherit all properties and methods of the class it extends. For example, class MySubClass extends MyClass will create a new class inherited from MyClass.

Can I override the method in a subclass of CoffeeScript?

Yes, you can override methods in a subclass of CoffeeScript. When you define a method in a subclass that has the same name as a method in the superclass, the subclass's method is used. If you still want to call the super class method, you can use the super keyword.

How to create an instance of a class in CoffeeScript?

To create an instance of a class in CoffeeScript, you can use the new keyword followed by the class name. For example, myInstance = new MyClass will create a new instance of MyClass.

What is the purpose of constructor methods in CoffeeScript class?

The constructor method in the CoffeeScript class is a special method that is called when a new instance of the class is created. It is usually used to initialize properties of a class. In CoffeeScript, the constructor method is defined using the constructor keyword.

Can I define private properties and methods in the CoffeeScript class?

CoffeeScript does not support private properties and methods in built-in. However, you can achieve similar effects by using closures or defining properties and methods in constructor methods, which makes them accessible only within instances of the class.

How to call a method on a class instance in CoffeeScript?

To call a method on a class instance in CoffeeScript, you can use the . operator followed by the method name. For example, myInstance.myMethod() will call the myMethod method on the myInstance object.

Can I add properties and methods to the class after defining the class in CoffeeScript?

Yes, you can add properties and methods to the class after defining the class in CoffeeScript. You can do this by defining them on the prototype of the class. For example, MyClass::myNewMethod = -> will add a new method to MyClass.

How to check the type of a class instance in CoffeeScript?

To check the type of a class instance in CoffeeScript, you can use the instanceof operator. For example, myInstance instance of MyClass Returns true if myInstance is an instance of MyClass.

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