Can you create nested classes in TypeScript?

Here is a more complex use case using class expressions.

It allows the inner class to access the private members of the outer class.

class classX { 
    private y: number = 0; 

    public getY(): number { return this.y; }

    public utilities = new class {
        constructor(public superThis: classX) {
        }
        public testSetOuterPrivate(target: number) {
            this.superThis.y = target;
        }
    }(this);    
}

const x1: classX = new classX();
alert(x1.getY());

x1.utilities.testSetOuterPrivate(4);
alert(x1.getY());

codepen

In modern TypeScript we have class expressions which you can use to create a nested class. For example you can do the following :

class Foo {
    static Bar = class {
        
    }
}

// works!
var foo = new Foo();
var bar = new Foo.Bar();

If you're in the context of a type declaration file, you can do this by mixing classes and namespaces:

// foo.d.ts
declare class Foo {
  constructor();
  fooMethod(): any;
}

declare namespace Foo {
  class Bar {
    constructor();
    barMethod(): any;
  }
}

// ...elsewhere
const foo = new Foo();
const bar = new Foo.Bar();

I couldn't get this to work with exported classes without receiving a compile error, instead I used namespaces:

namespace MyNamespace {
    export class Foo { }
}

namespace MyNamespace.Foo {
    export class Bar { }
}