What is the purpose of a `getInstance()` method?

Question

I am looking at a user service, my understanding is it's similar to a user service in Nest, but not really.

In it I see the following:

export class UsersService {
  private usersDao: UsersDao

  constructor() {
     this.usersDao = UsersDao.getInstance();
  }
}

static getInstance(): UsersService {
  if (!UsersService.instance) {
    UsersService.instance = new UsersService();
  }
  return UsersService.instance;
}

What is that getInstance() doing exactly? And why not just:

export class UsersService {

  constructor(private usersDao: UsersDao) {}
}

What is the goal of getInstance()?

The code is using the [Singleton pattern](https://www.dofactory.com/javascript/design-patterns/singleton) to limit the number of instances created by the code — blurfus, Nov 03 '21 at 17:28
That `static getInstance(): UsersService` method definition seems to live outside of any `class`. Can you provide a complete code, please? — Bergi, Nov 03 '21 at 19:28
Are you talking about `UsersDao.getInstance()` or about the other one (presumably `UsersService.getInstance`)? — Bergi, Nov 03 '21 at 19:29
@blurfus …and it's not even a proper singleton pattern, since everyone can still do `new UsersService();`. — Bergi, Nov 03 '21 at 19:33
@Bergi, interesting point. What would a proper singleton pattern in this context look like? — Daniel, Nov 03 '21 at 19:34
@blurfus, so you are suggesting that it still needs `private usersDao: UsersDao` inside the constructor argument list? — Daniel, Nov 03 '21 at 19:36
@Daniel It would put the `if (instance) return instance` right inside the constructor. Or even better, it wouldn't use `class` syntax and constructors at all, just a export a single object literal. — Bergi, Nov 03 '21 at 19:40
@Daniel, No, I mean the constructor itself can be marked as private - i.e. `private contructor() {...}` - see https://stackoverflow.com/a/40034649/600486 for a complete example — blurfus, Nov 03 '21 at 19:40

score 2 · Accepted Answer · edited Nov 03 '21 at 19:32

Usually this is part of the singleton pattern. Basically one class that, once instantiated, any subsequent classes will refer to that instance, rather than creating a fresh instance each time.

https://en.wikipedia.org/wiki/Singleton_pattern

Its useful for a class where something complex needs to happen when it is first constructed, but all following calls just need access to the properties.

_{I'd also like to mention that you can (in JavaScript specifically) export an instance, and all modules that import the module will have access to the same instance.}

What is the purpose of a `getInstance()` method?

1 Answers1