set readonly field from method not the constructor - typescript

Question

I have this class

class Foo { 

     private readonly serviceA : AService;
     private readonly serviceB : BService;
     private readonly serviceC : CService;
     ...

    constructor() {
        this.setupService();
        ...
    }


    private setupService() : void {
         this.serviceA = new ServiceA(...);
         this.serviceB = new ServiceB(...);
         ...
    }

}

as you can see I would like to set the fields using setupServices method which will be called from the contractor only. of course, this will fail to compile but I'm wondering if there is a way to get around this.

the reason I want this to clean up the contractor.

I could do this

constructor() {
    this.serviceA = new AService(...);
    this.serviceB = new BService(...);
    ...
}

but this will make a mass when I have many services

Using a class decorator may be an alternative. I don't know if it is a more elegant solution but it might keep your class neater. — Eldar
– Eldar, Commented Oct 30, 2020 at 18:24

CertainPerformance · Accepted Answer · 2020-10-30 17:49:24Z

1

You can use class fields instead:

class Foo { 
    private readonly service1 = new Service1();
    private readonly service2 = new Service2();
    private readonly service3 = new Service3();
}

As a bonus, this also removes the need for explicit type annotations.

but this will make a mass when I have many services

If you have a large number, consider using an array instead, which will probably be a bit nicer to work with than a whole lot of individual property names. Without class fields, this could look something like:

class Foo {
  // type could be made more precise via tuple if needed
  private readonly services: ServiceObj[];
  constructor() {
    this.services = this.makeServices();
  }
  makeServices() {
    // create and return array of services
  }
}

edited Oct 30, 2020 at 17:49

answered Oct 30, 2020 at 17:45

CertainPerformance

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1 Comment

Ali Faris Over a year ago

thank you for answering, your suggestion is very good. in the case of using the array, I think that will not be useful since the services of different type, and I also should have const index values for every service that I would like to use

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