ImmutableList in typescript

ImmutableList.ts

abstract class ImmutableList<T> {
    abstract add(elem: T): ImmutableList<T>
    abstract forEach(fn: (x: T) => void): void
}

class Cons<T> implements ImmutableList<T> {
    readonly el: T
    readonly next: ImmutableList<T>

    constructor(elem: T, list: ImmutableList<T> = Nil) {
        this.el = elem
        this.next = list
    }
    
    add(elem: T): ImmutableList<T> {
        return new Cons(elem, this)
    }

    forEach(fn: (x: T) => void): void {
        fn(this.el)
        this.next.forEach(fn)
    }
}

class EmptyList extends ImmutableList<never> {
    add(elem: never): ImmutableList<never> {
        return this
    }

    forEach(fn: (x: never) => void): void {}
}

const Nil = new EmptyList()

let list: ImmutableList<number> = new Cons(1)

list = list.add(2)
list = list.add(3)

list.forEach(console.log)