A static method written in Typescript within an abstract class for generating a new instance of the class itself

Question

A static method written in Typescript within an abstract class for generating a new instance of the class itself

Imagine I have

abstract class Foo {

}

class Bar1 extends Foo {
    constructor(someVar) { ... }
}

class Bar2 extends Foo {
    constructor(someVar) { ... }
}

I want to create a static method that generates an instance of the final class (all constructors having the same signature). Here's what I would like:

abstract class Foo {
    public static generateInstance(someVar) {
        const self = new this(someVar);
    }
}

However, because Foo is abstract, this seems impossible. Is there any way around this?

UPDATE

What if these classes use their own templates?

abstract class Foo<M> {

}

class Bar1 extends Foo<SomeModel> {...}

Now, I want the generateInstance method to be aware of the type SomeModel. So, I attempted

public static generateInstance<A, T extends Foo<A>>(this: new (someVar: any) => T, someVar: any): T {
    const self = new this(someVar);
    return self;
  }

Unless I explicitly call

Bar1.generateInstance<SomeModel>("blah")

, the returned result does not infer the data type. In other words, Bar1.generateInstance("blah") doesn't recognize the data type.

javascript typescript

Answer 1

Answer №1

To enhance the this type of a static method, it is possible to add an annotation. By doing so, this will point to the class itself. Adding an annotation for the this parameter means that the static method will be visible only on classes that meet certain criteria (such as having a constructor with a single argument). Additionally, this annotation helps in determining the actual instance type of the class where the method is called:

abstract class Foo {
  public static someAction<T extends Foo>(this: new (someVar: any) => T, someVar: any): T {
    const self = new this(someVar);
    return self;
  }
}

class Bar1 extends Foo {

  constructor(someVar: any) {
    super();
  }
}

class Bar2 extends Foo {
  constructor(someVar: any) {
    super();
  }
}
let bar1 = Bar1.someAction(0) // of type Bar1
let bar2 = Bar2.someAction(0) // of type Bar2

Answer 2

To enhance the this type of a static method, it is possible to add an annotation. By doing so, this will point to the class itself. Adding an annotation for the this parameter means that the static method will be visible only on classes that meet certain criteria (such as having a constructor with a single argument). Additionally, this annotation helps in determining the actual instance type of the class where the method is called:

abstract class Foo {
  public static someAction<T extends Foo>(this: new (someVar: any) => T, someVar: any): T {
    const self = new this(someVar);
    return self;
  }
}

class Bar1 extends Foo {

  constructor(someVar: any) {
    super();
  }
}

class Bar2 extends Foo {
  constructor(someVar: any) {
    super();
  }
}
let bar1 = Bar1.someAction(0) // of type Bar1
let bar2 = Bar2.someAction(0) // of type Bar2

Answer 3

Answer №2

One effective solution to this issue is to implement the factory method pattern, transferring the static method from the abstract class to the factory class.

This approach involves:

A factory class containing the static method responsible for generating specific classes.
An abstract class with shared data, logic, and necessary abstract components for implementation.
Extending classes that implement the abstract properties and methods inherited from the parent class.

Initially, an abstract class holds the shared data:

abstract class Foo{
  constructor(someVar: any){
    ...
  }
}

Followed by classes extending the abstract class:

class Bar1 extends Foo {
  constructor(someVar: any){
    super(someVar)
  }
}

class Bar2 extends Foo {
  constructor(someVar: any){
    super(someVar)
  }
}

To create instances of these classes based on parameters, a factory with creational logic simplifies the process:

class BarFactory {
  public static barCreator(someVar: any): Bar1 | Bar2 {
    if(someVar.x === 'whatever'){
      return new Bar1(someVar)    
    } else {
      return new Bar2(someVar)  
    }
  }
}

Finally, retrieve and utilize the instance using the factory:

const bar = BarFactory.barCreator(someVar)

All abstract properties and methods are readily accessible. However, to utilize specific methods or properties unique to the instantiated class, checking the instance type becomes necessary:

if (bar instanceof Bar1){
  ...
} else {
  ...
}

Answer 4

One effective solution to this issue is to implement the factory method pattern, transferring the static method from the abstract class to the factory class.

This approach involves:

A factory class containing the static method responsible for generating specific classes.
An abstract class with shared data, logic, and necessary abstract components for implementation.
Extending classes that implement the abstract properties and methods inherited from the parent class.

Initially, an abstract class holds the shared data:

abstract class Foo{
  constructor(someVar: any){
    ...
  }
}

Followed by classes extending the abstract class:

class Bar1 extends Foo {
  constructor(someVar: any){
    super(someVar)
  }
}

class Bar2 extends Foo {
  constructor(someVar: any){
    super(someVar)
  }
}

To create instances of these classes based on parameters, a factory with creational logic simplifies the process:

class BarFactory {
  public static barCreator(someVar: any): Bar1 | Bar2 {
    if(someVar.x === 'whatever'){
      return new Bar1(someVar)    
    } else {
      return new Bar2(someVar)  
    }
  }
}

Finally, retrieve and utilize the instance using the factory:

const bar = BarFactory.barCreator(someVar)

All abstract properties and methods are readily accessible. However, to utilize specific methods or properties unique to the instantiated class, checking the instance type becomes necessary:

if (bar instanceof Bar1){
  ...
} else {
  ...
}

Answer 5

Answer №3

After some experimenting, I discovered a solution to your issue with generics.

The trick is to avoid using a type variable in the extends constraint for the base class but rather include the base class with the type variable as an intersection of the return type.

Instead of T, utilize any within

static someAction<A, T extends Foo<any>>

and append & Foo<A> to the return type.

abstract class Foo<M> {...}

class Bar1 extends Foo<SomeModel> {...}

public static someAction<A, T extends Foo<any>>(
    this: new (someVar: any) => T, someVar: any
): T & Foo<A> {
    return new this(someVar);
}

I encountered the same problem when trying to create a user-defined type guard, and applying the aforementioned technique successfully resolved the issue.

public static isThisType<A, T extends Foo<any>>(
    this: new (...args: any) => T, value: unknown
): value is T & Foo<A> {
    return value instanceof this;
}

Answer 6

After some experimenting, I discovered a solution to your issue with generics.

The trick is to avoid using a type variable in the extends constraint for the base class but rather include the base class with the type variable as an intersection of the return type.

Instead of T, utilize any within

static someAction<A, T extends Foo<any>>

and append & Foo<A> to the return type.

abstract class Foo<M> {...}

class Bar1 extends Foo<SomeModel> {...}

public static someAction<A, T extends Foo<any>>(
    this: new (someVar: any) => T, someVar: any
): T & Foo<A> {
    return new this(someVar);
}

I encountered the same problem when trying to create a user-defined type guard, and applying the aforementioned technique successfully resolved the issue.

public static isThisType<A, T extends Foo<any>>(
    this: new (...args: any) => T, value: unknown
): value is T & Foo<A> {
    return value instanceof this;
}

A static method written in Typescript within an abstract class for generating a new instance of the class itself

Answer №1

Answer №2

Answer №3

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