Implement a simple singleton function in TypeScript

singleton.ts

const instances = new WeakMap();

// Create a singleton instance.
export function singleton<T>(fn: () => T): () => T {
    return () => {
        let instance = instances.get(fn);

        if (!instance) {
            instance = fn();
            instances.set(fn, instance);
        }

        return instance;
    };
}

// Usage
// get the redis client
export const getRedis = singleton(
    () => new IORedis()
);

const redis = getRedis();