Singletons in TypeScript

I’m not going to get in to the debate of why and if you should use singletons, i’m quite yet figuring that out myself.. But i am working on a legacy project written in OOP-style TypeScript and it has about a hundred singleton classes. So, naturally, i wanted to get up to speed with them. Anyhoo..

and here’s how you create a singleton in TypeScript

A singleton is a class that must only ever have one single instance of it and no more. Basically, you block anyone from creating another instance of it.

Static members don’t need an instance of a class in order to be used. You can call any static member from any class without actually instantiating that class. If you make properties and methods static then we don’t really need an instance of the class in order to use the functionality, do we?

But even if we have static members inside a class, the class itself can still be instantiated with new. How do we stop people from creating instances of it?

The old, standard code looks like this:

 1export default class MessageService extends ServiceBase {
 2  private static _instance: MessageService = new MessageService()
 3
 4  constructor() {
 5    super()
 6
 7    // throw an error if someone tries to use `new` to create an instance, tell them to use the `getInstance()` method instead
 8    if (MessageService._instance) {
 9      throw new Error('Error: Instantiation failed: Use MessageService.getInstance() instead of new.')
10    }
11
12    MessageService._instance = this
13  }
14
15  static getInstance(): MessageService {
16    return MessageService._instance
17  }
18}

Here’s how you’ll use it

1messageService: MessageService = MessageService.getInstance()

Each new MessageService operation will return the same instance. We also added a user friendly message to let the users of this class to know that it is a singleton and they can not instantiate this class with new and should use getInstance() that we have provided to them.

Another way of doing it is to make the constructor() private.

Changing the scope of a constructor to private removes our ability to use the new keyword.

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