Typescript: How to get subclass instance using parent get method

Question

Suppose I have a parent class with a static method that returns an instance,

abstract class AbstractManager {
    private static instance: AbstractManager;

    constructor() {
        AbstractManager.instance = this;
    }

    public static getInstance(): AbstractManager {
        return this.instance
    }

}

And I have a child class which extends the parent class,

class Manager extends AbstractManager {
    constructor() {
        super();
    }
}

If I invoke Manager.getInstance() the return type is of the parent AbstractManager.

Is there a way to set up the parent getInstance() method so that it returns the child type instead?

Obviously, I can add a getInstance() to each of the children class returning or casting to the child type but should I have 100 subclasses then I'll need to write the method 100 times.

Although my knowledge of generics is limited, another solution I have attempted using generics is,

public static getInstance<T extends AbstractManager>(): T {
    return this.instance;
}

But this has caused the error Type 'AbstractManager' is not assignable to type 'T'.

Is it possible to set it up such that

Manager.getInstance() // is of type Manager not AbstractManager.

Or should I approach this problem in a different way? Thank you.

distante · Accepted Answer · 2019-02-14 10:37:33Z

1

Your getInstance method is a static method of AbstractManager class, it does not do anything with instantiated classes. remove the static.

Also you do not need the instance member.

abstract class AbstractManager {

    constructor() {
    }

    public getInstance(): this {
        return this
    }

}

class Manager extends AbstractManager {
    managerTalk() {
        console.log("Hello I am Manager");
    }
    constructor() {
        super();
    }
}

const manager = new Manager();

const instance = manager.getInstance();
console.log(manager.getInstance()) // <--Manager

manager.managerTalk();
instance.managerTalk();

Take a look in Typescript Playground

edited Feb 14, 2019 at 10:37

answered Feb 14, 2019 at 9:25

distante

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6 Comments

aXises Over a year ago

Hi, thanks for the reply, the point of the static method is so I can retrieve the same instance of the class. In your solution, the manager.getInstance() method still has a return type of AbstractManager, not the child type Manager in visual studio code.

distante Over a year ago

@aXises no it is not. Take a look at the link in my updated answer.

aXises Over a year ago

add any instance method to the child class and the typescript playground will also display an error stating that the property does not exist on type AbstractManager. I have taken a screenshot since the playground link does not fit here. Image

distante Over a year ago

@aXises that was my fault, I forgot to remove the AbstractManager from the return type of getInstance to Typescript got the type wrong (but it was just a compilation issue). Answer updated.

aXises Over a year ago

Great, setting the return type to this will also work. Is it possible to set it up a way such that it can be invoked from a static context? This way the same instance can be used and retrieved by using the static getInstance() method.

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