How to Define a Method in a Class that Returns Only Part of the Class Using TypeScript

Learn how to create methods in TypeScript classes that selectively return properties and methods, enhancing encapsulation and type safety in your applications.
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Understanding the Problem

When working with classes in TypeScript, you may encounter situations where you want to expose only certain methods or properties while keeping others private. This is particularly useful in ensuring encapsulation and a cleaner interface. A common scenario arises when dealing with a class method that needs to return only its function properties, excluding any other attributes or methods that should remain hidden.

For instance, consider this class structure:

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Solution Overview

To restrict what a method returns, you can use TypeScript's features like private, protected, and utility types. These approaches help you create a class method that can return only a part of the class, specifically its function properties while hiding other attributes.

Step 1: Utilizing private and abstract

Make certain properties and methods private or protected to prevent them from being exposed outside the class.

Use abstract methods to define what functionality must be implemented by subclasses.

Here’s how this can be applied:

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The word property is now private, meaning it can't be accessed outside the Service class.

Step 2: Implementing Inheritance with the Derived Class

Next, we will create a subclass implementing the required methods:

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The hello and greet methods can be called as expected, but attempting to access word will result in an error.

Step 3: Advanced Types with Utility Functions

If you want to restrict the instance type dynamically to include only the functions of the class, you can leverage type utilities:

Defining a Utility Type

FunctionKeys

This type extracts keys from an interface where the property type is a function.

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Creating a Service Instance

We can create a function that returns an instance restricted to only the function properties:

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Here, service will only have access to the hello and greet methods, enhancing encapsulation.

Conclusion

Using TypeScript, you can effectively control the visibility and accessibility of class methods and properties. By implementing private, protected, and dynamic typing constructs, you can build robust and maintainable code while ensuring that your classes expose only the methods that are necessary for external interaction.

In summary, this approach not only enforces encapsulation but also encourages cleaner APIs within your TypeScript applications.