How to define Singleton in TypeScript

Question

What is the best and most convenient way to implement a Singleton pattern for a class in TypeScript? (Both with and without lazy initialisation).

Answer 1

I have found a new version of this that the Typescript compiler is totally okay with, and I think is better because it doesn't require calling a getInstance() method constantly.

import express, { Application } from 'express';

export class Singleton {
  // Define your props here
  private _express: Application = express();
  private static _instance: Singleton;

  constructor() {
    if (Singleton._instance) {
      return Singleton._instance;
    }

    // You don't have an instance, so continue

    // Remember, to set the _instance property
    Singleton._instance = this;
  }
}

This does come with a different drawback. If your Singleton does have any properties, then the Typescript compiler will throw a fit unless you initialize them with a value. That's why I included an _express property in my example class because unless you initialize it with a value, even if you assign it later in the constructor, Typescript will think it hasn't been defined. This could be fixed by disabling strict mode, but I prefer not to if possible. There is also another downside to this method I should point out, because the constructor is actually getting called, each time it does another instance is technically created, but not accessible. This could, in theory, cause memory leaks.