singleton-decorator.ts

type Class<Instance extends any> = (new (...args: any[]) => Instance) & { [key: string]: any }

function MySingleton() {
    return function decorator<Instance extends any>(constructor: Class<Instance>) {
        let instance: Instance | null = null

        let decoratedConstructor = function (...args: any[]) {
            if (!instance) {
                instance = new constructor(...args)
            }
            return instance
        } as Function as Class<Instance>

        decoratedConstructor.prototype = constructor.prototype
        return decoratedConstructor as Class<Instance>
    }
}