Type-safe singleton superclass

example.ts

import { Singleton } from './Singleton';

class MySingleton extends Singleton {
    public sayHello() {
        console.log('Hello, world!');
    }
}

console.log(MySingleton.getInstance() === MySingleton.getInstance());
// > true

MySingleton.getInstance().sayHello();
// > Hello, world!

Singleton.ts

/**
 * A superclass for singleton classes. If constructor arguments are needed a single call to `new ClassName(...args)` is
 * possible, but only before using `ClassName.getInstance()`. Attempting to create additional instances will throw an
 * error.
 */
export abstract class Singleton {
    /**
     * Retrieves the singleton instance.
     */
    public static getInstance<T extends Singleton>(this: { [key: string]: any; new (): T }): T {
        return this._instance || new this();
    }

    private static _instance: Singleton;

    constructor() {
        const ctor = <typeof Singleton>this.constructor;

        if (ctor._instance) {
            const name = ctor.name;
            throw new Error(`${name} is a singleton class. Use ${name}.getInstance() instead.`);
        }

        ctor._instance = this;
    }
}