This is a singleton class implementation using `constructor()` in TypeScript.

Singleton.ts

import crypto from 'node:crypto'

class Singleton {
    private static instance?:Singleton|null

    constructor() {
        if (Singleton.instance) return Singleton.instance
        
        if (Singleton.instance === undefined) {
            Singleton.instance = null
            Singleton.instance = new Singleton()
            return Singleton.instance
        }
    }

    uuid = crypto.randomUUID()

    sayHello() { console.log('hello world', this.uuid) }
}

const one = new Singleton()
const two = new Singleton()
one.sayHello() // hello world e6d5790d-e960-445e-9774-71d0cca7548f 🔥
two.sayHello() // hello world e6d5790d-e960-445e-9774-71d0cca7548f 🔥