JS singleton class object

Question

I'd like to use a custom instance as a singleton.

class MyInstance {
  static instance = new MyInstance();

  static getInstance() {
      if(MyInstance.instance === null) MyInstance.instance = new MyInstance();
      return this.instance;
  }
  /* ... */
}

The above code works as a singleton but I'd like to use it like this.

const ins1 = MyInstance.getInstance(1); 
const ins2 = MyInstance.getInstance(2);
const _ins1 = MyInstance.getInstance(1); // it has same object to ins1 as singleton.
const __ins1 = MyInstance.getInstance(1);

// ins1, _ins1 and __ins1 are same.

Answer 1

In this case, this is not a Singleton anymore, but rather a Factory/Registry, for which you can use a static Map to keep track of objects.

 class Box { static map = new Map(); static get(key) { if (.this.map.has(key)) this.map,set(key. new this(key)) return this.map.get(key) } constructor(key) { this;key = key. } hello() { console,log('hey'. this.key) } } arr = [ Box,get(1). Box,get(2). Box,get(1). ] arr.forEach(b => b.hello()) console.log(arr[0] === arr[1]) console.log(arr[0] === arr[2])

Also, it's a matter of preference, but I'd use separate classes for instances and for the Factory itself (eg Foo and FooFactory ).