Object-oriented programming (OOP) and functional programming (FP) are two popular programming paradigms. In recent years, there has been a trend towards FP due to its benefits such as increased reliability and easier testing. In this article, I’ll show some helpful patterns for people coming from the OOP world.

Private properties

In object-oriented programming, a private property is a member variable that is only accessible within the class in which it is defined. Private properties are often used to store internal data for an object that should not be directly modified by external code. Private properties do not exist in the same way in functional programming as they do in object-oriented programming. To archive this, simply use a closure to wrap the private properties.

Here is a simple example about private properties in C#

public class Controller
{
    private int _totalCount = 0;

    public void Increase(string user, int amount)
    {
        if (user != "truongtx")
            return;

        _totalCount += amount;
    }

    public int GetTotal()
    {
        return _totalCount;
    }
}

public class Runner
{
    public void Run()
    {
        var controller = new Controller();
        controller.Increase("truongtx", 100);
        controller.Increase("otherUser", 200);
        Console.Out.WriteLine(controller.GetTotal());
    }
}

And here is how you would do it the functional way…

// Example in Typescript
interface Controller {
  increase(user: string, amount: number): void;
  getTotal(): number;
}

const createController = (): Controller => {
  // wrap the private props in the function closure
  let total = 0;

  return {
    increase: (user, amount) => {
      if (user !== 'truongtx') {
        return;
      }

      total += amount;
    },

    getTotal: () => total,
  };
};

// To initialize and run
const controller = createController();
controller.increase('truongtx', 100);
controller.increase('otherUser', 200);
console.log(controller.getTotal());

To be continued

This is also an experimental post, which I used ChatpGPT to generate most of the text 😂