Understanding the Difference Between Interfaces and Classes in Typescript

Welcome to the “Understanding the Difference Between Interfaces and Classes in TypeScript” section of our course “Learn TypeScript”! In this article, we will cover the differences between interfaces and classes in TypeScript, and how you can use them to write more structured and reusable code. By the end of this article, you should have a good understanding of these concepts and be able to use them effectively in your TypeScript programs.

Introduction to Interfaces

Interfaces in TypeScript are used to describe the shape of an object. They define a set of properties and methods that an object should have, but do not provide any implementation for those properties and methods. For example:

interface Point {
  x: number;
  y: number;
  distanceToOrigin(): number;
}

In this example, we have defined an interface called “Point” that has two properties, “x” and “y”, and one method, “distanceToOrigin”. The interface specifies that an object that implements this interface must have these properties and method, but does not specify how they should be implemented.

To use an interface, you can create a class or object that implements it. For example:

class CartesianPoint implements Point {
  x: number;
  y: number;

  constructor(x: number, y: number) {
    this.x = x;
    this.y = y;
  }

  distanceToOrigin() {
    return Math.sqrt(this.x * this.x + this.y * this.y);
  }
}

In this example, we have defined a class called “CartesianPoint” that implements the “Point” interface. The class defines the properties and method specified in the interface and provides an implementation for them.

Introduction to Classes

Classes in TypeScript are used to define objects that have both state and behavior. They provide a way to organize and structure your code, and can be used to create objects with specific properties and methods. For example:

class Point {
  x: number;
  y: number;

  constructor(x: number, y: number) {
    this.x = x;
    this.y = y;
  }

  distanceToOrigin() {
    return Math.sqrt(this.x * this.x + this.y * this.y);
  }
}

In this example, we have defined a class called “Point” that has two properties, “x” and “y”, and one method, “distanceToOrigin”. The class defines the structure and behavior of the “Point” object, and provides an implementation for the properties and method.

To use a class, you can create an instance of the class using the “new” operator. For example:

let point = new Point(3, 4);
console.log(point.distanceToOrigin()); // Outputs: 5

In this example, we have created an instance of the “Point” class called “point”, and called the “distanceToOrigin” method on it.

Differences Between Interfaces and Classes

While both interfaces and classes are used to define the structure and behavior of objects in TypeScript, they have some key differences:

• Implementation: Interfaces define the structure of an object, but do not provide any implementation. Classes, on the other hand, define both the structure and the implementation of an object.
• Inheritance: Classes can inherit from other classes, using the “extends” keyword. Interfaces, on the other hand, cannot inherit from other interfaces, but can extend multiple interfaces using the “extends” keyword.
• Modifiers: Classes can have different levels of accessibility, using the “public”, “private”, and “protected” modifiers. Interfaces, on the other hand, do not have any accessibility modifiers and all members are automatically public.

When to Use Interfaces vs Classes

So, when should you use interfaces vs classes in TypeScript? The answer depends on your specific needs and goals. Here are a few guidelines to help you decide:

• Use interfaces when you want to define the structure of an object, but do not need to provide any implementation. Interfaces are useful when you want to specify a contract that other classes or objects must adhere to.
• Use classes when you want to define both the structure and the implementation of an object. Classes are useful when you want to create objects with specific properties and methods that you can reuse in your code.
• Use inheritance when you want to create a class that is a specialized version of another class. Inheritance is useful when you want to share common code between classes, but also add some specific behavior or properties.
• Use interfaces when you want to extend multiple classes or interfaces. Interfaces are useful when you want to define a complex object that has multiple inheritance chains.

Conclusion

In conclusion, this article has covered the differences between interfaces and classes in TypeScript, and how you can use them to write more structured and reusable code. You should now have a good understanding of these concepts and be able to use them effectively in your TypeScript programs. In future sections of our course, we will cover more advanced topics, such as generics and decorators, and how to use them to write more powerful TypeScript code.

Exercises

To review these concepts, we will go through a series of exercises designed to test your understanding and apply what you have learned.

class Point {
  x: number;
  y: number;
}

class CartesianPoint extends Point {
  // CartesianPoint inherits from Point
}

interface Point {
  x: number;
  y: number;
}

interface ThreeDimensionalPoint extends Point {
  z: number;
}