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

Another option is to use Symbols in your module. This way you can protect your class, also if the final user of your API is using normal Javascript:

let _instance = Symbol();
export default class Singleton {

    constructor(singletonToken) {
        if (singletonToken !== _instance) {
            throw new Error("Cannot instantiate directly.");
        }
        //Init your class
    }

    static get instance() {
        return this[_instance] || (this[_instance] = new Singleton(_singleton))
    }

    public myMethod():string {
        return "foo";
    }
}

Usage:

var str:string = Singleton.instance.myFoo();

If the user is using your compiled API js file, also will get an error if he try to instantiate manually your class:

// PLAIN JAVASCRIPT: 
var instance = new Singleton(); //Error the argument singletonToken !== _instance symbol