How to Create Nested Classes in TypeScript ?

In TypeScript, you can create nested classes using different methods. We will discuss about three different approaches to creating nested classes in TypeScript.

These are the approaches:

By defining nested classes inside a class

In this approach, we will directly define the nested class inside the outer class.

Syntax:

class OuterClass{
    static InnerClass = class{
    }
}

Example: The below code example defines a nested class inside another class.

Javascript

class OuterClass {
    outerClassProperty: string;

    constructor(outerClassProperty: string) {
        this.outerClassProperty = outerClassProperty;
    }

    outerClassMethod() {
        console.log
            (`Outer Class property:
            ${this.outerClassProperty}`);
    }

    // Nested class
    static InnerClass = class {
        innerClassProperty: string;

        constructor(innerClassProperty: string) {
            this.innerClassProperty = innerClassProperty;
        }

        innerClassMethod() {
            console.log
                (`Inner Class property:
                ${this.innerClassProperty}`);
        }
    }
}
const outerClassInstance = new OuterClass("GeeksforGeeks");
outerClassInstance.outerClassMethod();
const innerClassInstance =
    new OuterClass.InnerClass("A Computer Science Portal");
innerClassInstance.innerClassMethod();

Output:

Outer Class property: GeeksforGeeks
Inner Class property: A Computer Science Portal

By using the namespaces

The namespace can also be used to create a namespace of Outer class and the namespace of the inner class which can be later merged together to create nested classes.

Syntax:

namespace namespace_name{
    export class OuterClass{}
}
namespace namespace_name.OuterClass{
    export class InnerClass{}
}

Example: The below code example will help you to create the nested classes using the namespace approach.

Javascript

namespace nestedClasses {
    export class OuterClass {
        outerClassProperty: string;

        constructor(outerClassProperty: string) {
            this.outerClassProperty = outerClassProperty;
        }

        outerClassMethod() {
            console.log
                (`Outer Class property:
                 ${this.outerClassProperty}`);
        }
    }
}

namespace nestedClasses.OuterClass {
    export class InnerClass {
        innerClassProperty: string;

        constructor(innerClassProperty: string) {
            this.innerClassProperty = innerClassProperty;
        }

        innerClassMethod() {
            console.log
                (`Inner Class property:
                 ${this.innerClassProperty}`);
        }
    }
}

const outerClassInstance =
    new nestedClasses.OuterClass("GeeksforGeeks");
outerClassInstance.outerClassMethod();
const innerClassInstance =
    new nestedClasses.OuterClass
        .InnerClass("A Computer Science Portal");
innerClassInstance.innerClassMethod();

Output:

Outer Class property: GeeksforGeeks 
Inner Class property: A Computer Science Portal

By using the modules

The modules can also be used to create nested classes in TypeScript in the same way as we used namespace.

Syntax:

namespace namespace_name{
     export class OuterClass{} 
} 
namespace namespace_name.OuterClass{
     export class InnerClass{} 
}

Example: The below example uses the modules to define the nested classes in TypeScript.

Javascript

module nestedClasses {
    export class OuterClass {
        outerClassProperty: string;

        constructor(outerClassProperty: string) {
            this.outerClassProperty = outerClassProperty;
        }

        outerClassMethod() {
            console.log
                (`Outer Class property:
             ${this.outerClassProperty}`);
        }
    }
}

module nestedClasses.OuterClass {
    export class InnerClass {
        innerClassProperty: string;

        constructor(innerClassProperty: string) {
            this.innerClassProperty = innerClassProperty;
        }

        innerClassMethod() {
            console.log
                (`Inner Class property:
            ${this.innerClassProperty}`);
        }
    }
}

const outerClassInstance =
    new nestedClasses.OuterClass("GeeksforGeeks");
outerClassInstance.outerClassMethod();
const innerClassInstance =
    new nestedClasses.OuterClass
        .InnerClass("A Computer Science Portal");
innerClassInstance.innerClassMethod();

Output:

Outer Class property: GeeksforGeeks 
Inner Class property: A Computer Science Portal