How to Extend abstract class with Generics in Typescript ?

In Typescript, an abstract class is the base class that is inherited by other classes without having to define its members. Generic is the feature with which you can create a variable that represents a type in classes, functions, and type aliases that don't need to define the types that they use explicitly.

There are 3 approaches given below to extend an abstract class with generics:

Using a basic way to extend the abstract class with Generics

In this approach, we will see the basic way to extend the abstract class with generics with 3 steps:

• Create a class with an abstract keyword and add a generic type parameter using angle brackets (<>).
• Use the generic type parameters in methods or properties within the abstract class.
• Extend the abstract class from other classes by providing types for the generic parameters.

Syntax:

abstract class Classname<Generic> {
    Visibility Prop_Name: Generic = Initial_Value;
    Method_Name(Parameter: Generic): Return_Type{ ... }
}
class NormalClass extends Abstract_Class<Generic_Type> { ... }

Example : The below example will create a simple abstract class SuperClass with a generic type parameter T. It has a method pushItem that accepts an item of type T and stores it in an array of type T[].

abstract class SuperClass<T> {
    protected NumArray: T[] = [];

    pushNumber(num: T): void {
        this.NumArray.push(num);
    }

    abstract ShowNumbers(): void;
}

class SubClass extends SuperClass<number> {
    showNumbers(): void {
        console.log("Pushed Numbers are: ", 
        this.NumArray.join(", "));
    }
}

const numberSeries = new SubClass();
numberSeries.pushNumber(1);
numberSeries.pushNumber(2);
numberSeries.showNumbers();

Output:

Pushed Numbers are: 1, 2

Extending with Generic Constraints

Extending with the use of Generic Constraints can be done in the following 4 steps:

• Create an interface of generic type with constraints
• Create an abstract class with a generic type parameter constrained by the above interface
• Extend the abstract class with a class that implements the interface
• Extend the abstract class with the normal class by which the normal class will have the power to pass a class as a type

Syntax:

interface Interface_Name{ ... }
abstract class AbstractClass<T extends Interface_Name> { ... }
class ClassNameOne implements Interface_Name{ ... }
class ClassNameTwo extends AbstractClass<ClassNameOne > { ... }

Example: The below example creates a class that extends an abstract class with passing another class as a generic and the generic also extends some interface.

interface MethodGateway {
    printName(): string;
}

abstract class AbstractClass<T extends MethodGateway> {
    abstract printName(): T;
}

class ClassMethodExtender implements MethodGateway {
    printName(): string {
        return "GeeksforGeeks";
    }
}

class NormalClass extends AbstractClass<ClassMethodExtender> {
    printName(): ClassMethodExtender {
        return new ClassMethodExtender();
    }
}

const instance = new NormalClass();
console.log(instance.printName().printName());

Output:

GeeksforGeeks

With multiple type parameters in Generics

Passing multiple Type parameters is explained in 2 steps below:

• Create an abstract class with multiple generic type parameters like T,U...
• Extend the abstract class with specific types

Syntax:

abstract class AbstractClass<T, U, ...> { ... }
class NormalClass extends AbstractClass<DataType_1, DataType_2, ...> { ... }

Example: The below example uses string type to pass to generic types that allows the abstract class to assign type to two abstract properties key and value.

abstract class KeyValue<Key, Value> {
    abstract key: Key;
    abstract value: Value;

    abstract display(): void;
}

class StringType extends KeyValue<string, string> {
    key: string = "name";
    value: string = "GeeksforGeeks";

    display(): void {
        console.log(`Key: ${this.key}, Value: ${this.value}`);
    }
}

const instance = new StringType();
instance.display(); 

Output:

Key: name, Value: GeeksforGeeks