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

The following approach creates a Singleton class that can be used exacly like a conventional class:

class Singleton {
    private static instance: Singleton;
    //Assign "new Singleton()" here to avoid lazy initialisation

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

        this. member = 0;
        Singleton.instance = this;
    }

    member: number;
}

Each new Singleton() operation will return the same instance. This can however be unexpected by the user.

The following example is more transparent to the user but requires a different usage:

class Singleton {
    private static instance: Singleton;
    //Assign "new Singleton()" here to avoid lazy initialisation

    constructor() {
        if (Singleton.instance) {
            throw new Error("Error - use Singleton.getInstance()");
        }
        this.member = 0;
    }

    static getInstance(): Singleton {
        Singleton.instance = Singleton.instance || new Singleton();
        return Singleton.instance;
    }

    member: number;
}

Usage: var obj = Singleton.getInstance();