If you need help on a project, please reach out, and let’s work together. TypeScript gives developers the ability to make an interface extend a class. Andrés Reales is the founder of Become a Better Programmer blogs and tutorials and Senior Full-Stack Software Engineer. With the this keyword, you can change typing dynamically in different class contexts.

When we compare two different types, regardless of where they came from, if the types of all members are compatible, then we say the types themselves are compatible.

So when I print this.name in the feed method, it points to the subclasses copy of name which is still set as “Emmy” by default. In example 1 above, the Cat and Dog properties, methods and constructors were created automatically behind the scenes. It was unnecessary to override there constructor, property values and feed method. They simply used whatever they got when they extended the Animal class.

Mistake and Correction During the Implementation of the Interface While Extending Class

They will now have preference over the equivalent properties in the base class if you refer to them using the this keyword. In TypeScript, interfaces can inherit classes using extends. It means that the interface can use the methods and properties of a class to create a prototype but cannot implement those. With abstract classes and interfaces, you are able to put together more complex type-checking for your classes to ensure that classes extended from base classes inherit the correct functionality. Next, you will run through examples of how method and property visibility work in TypeScript.

Can you extend two classes in TypeScript?

TypeScript classes cannot extend several classes at the same time unless a mixin is introduced to the interface. At this point, the only solution to this limitation is to introduce TypeScript mixins.

A constructor is a method that runs every time a new instance of the class is created. If the parent class constructor returns an object, that object will be used as the this value for the derived class when further initializing class fields. This trick Software Development Contracts is called “return overriding”, which allows a derived class’s fields to be defined on unrelated objects. While the base class may return anything from its constructor, the derived class must return an object or undefined, or a TypeError will be thrown.


It is also good to mention that changing static property is frowned upon, here greeter3 has “Hey there!” instead of “Hello, there” on standardGreeting. Parameter properties are declared by prefixing a constructor parameter with an accessibility modifier or readonly, or both. Using private for a parameter property declares and initializes a private member; likewise, the same is done for public, protected, and readonly. The Phone class is an implementation of the contract in the ImportedPhone interface that you created above. The foremost thing you should do is implement all the properties and methods from the interface. To improve this code, you could use a special type available inside classes, which is the this type.

typescript extend class

It means that the ImportedPhone interface can access the methods and properties defined in the RegularPhone interface. This offers a way for you to deliver code that is type-safe, more reliable, and that better represents the business model of your application. The highlighted code shows the identifier property declared with protected visibility. The this.identifier code tries to access this property from the FinanceEmployee subclass. When working with classes, most of the time you will need to create a constructor function.

JS Reference

Thereafter, two interfaces would be defined each containing separate methods declarations within them whose definitions will be defined laterwards within a separate class . In this example, the interface D extends the interfaces B and C. So D has all the methods of B and C interfaces, which are a(), b(), and c() methods. And you have many classes that already implemented the Mailable interface.

Can a class extend a class and implement an interface?

An interface can extend other interfaces, just as a class subclass or extend another class. However, whereas a class can extend only one other class, an interface can extend any number of interfaces. The interface declaration includes a comma-separated list of all the interfaces that it extends.

The members of a class (properties & methods) are typed using type annotations, similar to variables. The interface B extends the interface A, which then have both methods a() and b() . But sometimes we don’t want it because it created a new class named SubEmployee . If the Employee class is already used everywhere and by everybody, it’s not easy to update them all to the new subclass. Extending a class is a way of creating a class similar to another class but with additional functionality.

TypeScript – Classes

The extends keyword in TypeScript is used to implement inheritance, a class-based object-oriented characteristic that lets the child class or the interface acquire the members from their parents. The extends keyword also comes in handy while performing abstraction, which uses inheritance. In this example, first, we have defined the Human interface with fname, lName, and FullName. For your own classes, you will likely have to make the same decisions. However, for built-in classes, optimizability and security are a much bigger concern.

This series will show you the syntax you need to get started with TypeScript, allowing you to leverage its typing system to make scalable, enterprise-grade code. Argument of type ‘FinanceEmployee’ is not assignable to parameter of type ‘MarketingEmployee’. Remember that TypeScript is compiled to raw JavaScript that by itself does not have any way to specify the visibility of the members of a class.

typescript extend class

It means that when an interface extends a class with private or protected members, the interface can only be implemented by that class or subclasses of that class from which the interface extends. Abstract classes are base classes from which https://topbitcoinnews.org/ other classes may be derived. Unlike an interface, an abstract class may contain implementation details for its members. The abstract keyword is used to define abstract classes as well as abstract methods within an abstract class.

Private members are only accessible inside the class that declares them. In this example, the fileStream instance automatically has the copy method available on it. The FileStream class also had to implement a read and a write method explicitly to adhere to the Stream abstract class. TypeScript also has a shortcut for writing properties that have the same name as the parameters passed to the constructor. Instance methods try to delegate to a minimal set of primitive methods where possible. TypeScript provides a convenient way to define class members in the constructor, by adding a visibility modifiers to the parameter.

This means .map(Promise.resolve) throws an error because the this inside Promise.resolve is undefined. A way to fix this is to fall back to the base class if this is not a constructor, like Array.from() does, but that still means the base class is special-cased. Thereafter a derived class is created which is responsible for extending the parent class as well as the interfaces defined.

typescript extend class

As such, TypeScript has no protection against such usage during runtime. This is a safety check done by the TypeScript compiler only during compilation. Types of parameters ‘message’ and ‘message’ are incompatible. Property ‘instantiatedAt’ has no initializer and is not definitely assigned in the constructor. You will need sufficient knowledge of JavaScript, especially ES6+ syntax, such as destructuring, rest operators, and imports/exports.