Private class constructors in TypeScript

[robinpokorny]

While reading the TypeScript class documentation, I noticed that it is possible to mark any class member as private (or protected). And I wondered whether a constructor could be private, too. It turns out it can, and there is a valid use case for it!

First, what does a private constructor look like? Like this!

class MyClass {
  private constructor(private name: string) {}
}

const x = new MyClass();
//        ~~~~~~~~~~~~~
// Constructor of class 'MyClass' is private and only accessible within the class declaration.(2673)

As you can see, it's impossible to create a new instance of the class.

So, how is that useful? That class is basically worthless, right? Eeh, no!

The secret lies in the second part of the error message: ‘only […] within the class declaration’.

A little bit surprisingly to me, that also includes static methods. So you can create a class with explicit creation types.

An example would be a reimagination of the Date object, which is notoriously tricky:

class MyDate {
  private constructor(private timestamp: number) {}

  static fromTimestamp(timestamp: number): MyDate {
    return new MyDate(timestamp);
  }

  static fromTimestampMs(timestamp: number): MyDate {
    return new MyDate(timestamp * 1000);
  }

  static fromIsoString(iso: string): MyDate {
    return new MyDate(timestampFromIso(iso));
  }

  // ...
}

const today = MyDate.fromIsoString('2022-10-25T22:16:52.701Z')
//    ^? const today: MyDate

Now, I can clearly see some benefits in some use cases. It's a nice pattern to remember.

Oh, and of course, none of that limitation exists when the class is accessed from JavaScript. Be careful!