Utilizing generics within a function

Question

Utilizing generics within a function

Can you help me understand why I am getting an error message that says "Type 'AbstractPopup' is not assignable to type T" when using the return statement in the popupFactory(...) method? This code is just a test example for learning how generics work.

function popupFactory<T extends AbstractPopup>(popupType: PopupType, data: {}): T 
{
    var popup: AbstractPopup;

    switch (popupType)
    {

        case PopupType.ConcretePopup:
        {
                popup = new ConcretePopup();
                break;
        }
    }

    return popup;
}

abstract class AbstractPopup
{
    type: PopupType;
}

class ConcretePopup extends AbstractPopup{}

var p = popupFactory<ConcretePopup>(PopupType.ConcretePopup, {});

typescript

Answer 1

Answer №1

When discussing a generic function, it should be understood that it operates based on the type parameter provided to it. However, in this particular scenario, the method determines its return type according to the enum parameter rather than the generic parameter. As a result, the method cannot be considered truly generic.

The reason for the compiler error is due to the fact that, as any T derived from

AbstractPopup</code could be passed, there is no guarantee that the returned type will be compatible with the specific <code>T

being used in the function call. For example:

class OtherConcretePopup extends AbstractPopup{}
var p = popupFactory<OtherConcretePopup>(PopupType.ConcretePopup, {})

In this case, the code would technically be valid, but you are returning a ConcretePopup even though the expected return type is OtherConcretePopup.

A straightforward solution to this issue would be to create overloaded versions of the method for each different popup type:

function popupFactory(popupType: PopupType.OtherConcretePopup, data: {}): OtherConcretePopup
function popupFactory(popupType: PopupType.ConcretePopup, data: {}): ConcretePopup
function popupFactory(popupType: PopupType, data: {}): AbstractPopup 
{
    var popup: AbstractPopup = null;
    switch (popupType)
    {
        case PopupType.ConcretePopup:
        {
                popup = new ConcretePopup();
                break;
        }
    }

    return popup;
}

Alternatively, you could pass the constructor itself as an argument and eliminate the need for the switch statement entirely. This approach works best if all constructors have the same parameters and initialization process:

function popupFactory<T extends AbstractPopup>(popupType: new() => T, data: {}): T
{
    var popup = new popupType();
    return popup;
}

var p = popupFactory(ConcretePopup, {})

Answer 2

When discussing a generic function, it should be understood that it operates based on the type parameter provided to it. However, in this particular scenario, the method determines its return type according to the enum parameter rather than the generic parameter. As a result, the method cannot be considered truly generic.

The reason for the compiler error is due to the fact that, as any T derived from

AbstractPopup</code could be passed, there is no guarantee that the returned type will be compatible with the specific <code>T

being used in the function call. For example:

class OtherConcretePopup extends AbstractPopup{}
var p = popupFactory<OtherConcretePopup>(PopupType.ConcretePopup, {})

In this case, the code would technically be valid, but you are returning a ConcretePopup even though the expected return type is OtherConcretePopup.

A straightforward solution to this issue would be to create overloaded versions of the method for each different popup type:

function popupFactory(popupType: PopupType.OtherConcretePopup, data: {}): OtherConcretePopup
function popupFactory(popupType: PopupType.ConcretePopup, data: {}): ConcretePopup
function popupFactory(popupType: PopupType, data: {}): AbstractPopup 
{
    var popup: AbstractPopup = null;
    switch (popupType)
    {
        case PopupType.ConcretePopup:
        {
                popup = new ConcretePopup();
                break;
        }
    }

    return popup;
}

Alternatively, you could pass the constructor itself as an argument and eliminate the need for the switch statement entirely. This approach works best if all constructors have the same parameters and initialization process:

function popupFactory<T extends AbstractPopup>(popupType: new() => T, data: {}): T
{
    var popup = new popupType();
    return popup;
}

var p = popupFactory(ConcretePopup, {})

Answer 3

Answer №2

To avoid using a switch statement, you can utilize the following generic factory method to instantiate any derived class. The new () => T signature signifies a type with no argument constructor, allowing you to directly specify the name of the derived class if it has a default no argument constructor as shown below -

function popupFactory<T extends AbstractPopup>(popupType:  new () => T, data: {}): T 
{
    var newPopup: T;
    newPopup = new popupType();
    return newPopup;
}

abstract class AbstractPopup
{
    //type: PopupType;
}


class ConcretePopup extends AbstractPopup{  
}

var p = popupFactory(ConcretePopup, {}); // creates instance of ConcretePopup class

However, when your derived class possesses one or more parameters in its constructor and you need to provide parameter values during the instantiation of the class, you can adopt the approach outlined below -

function popupFactory<T extends AbstractPopup>(popupType:  new (obj:Object) => T, data: {}): T 
{
    var newCust: T;
    newCust = new popupType(data);
    return newCust;
}

abstract class AbstractPopup
{
    //type: PopupType;
}


class ConcretePopup extends AbstractPopup{
    public fname: string;
    public lname: string;
    constructor(obj:Object) {
        super();
        for (let key in obj) {
            if (obj.hasOwnProperty(key)) {
                if (key == "fname") {
                    this.fname = obj["fname"];
                }
                if (key == "lname") {
                    this.lname = obj["lname"];
                }
            }
        }       

    }
}
var p = popupFactory(ConcretePopup, {"fname":"Niladri","lname":"D"});
console.log(p.fname); // Niladri
console.log(p.lname); // D

In this scenario, I am passing

popupType:  new (obj:Object) => T

as a type that requires an Object as a parameter in its constructor. Thus, we can supply the necessary properties and their corresponding values as an object through the data parameter. However, the drawback is having to manually extract properties for the derived class as demonstrated in the constructor of the ConcretePopup class above.

You can access a live example by clicking on this working fiddle link provided.

Answer 4

To avoid using a switch statement, you can utilize the following generic factory method to instantiate any derived class. The new () => T signature signifies a type with no argument constructor, allowing you to directly specify the name of the derived class if it has a default no argument constructor as shown below -

function popupFactory<T extends AbstractPopup>(popupType:  new () => T, data: {}): T 
{
    var newPopup: T;
    newPopup = new popupType();
    return newPopup;
}

abstract class AbstractPopup
{
    //type: PopupType;
}


class ConcretePopup extends AbstractPopup{  
}

var p = popupFactory(ConcretePopup, {}); // creates instance of ConcretePopup class

However, when your derived class possesses one or more parameters in its constructor and you need to provide parameter values during the instantiation of the class, you can adopt the approach outlined below -

function popupFactory<T extends AbstractPopup>(popupType:  new (obj:Object) => T, data: {}): T 
{
    var newCust: T;
    newCust = new popupType(data);
    return newCust;
}

abstract class AbstractPopup
{
    //type: PopupType;
}


class ConcretePopup extends AbstractPopup{
    public fname: string;
    public lname: string;
    constructor(obj:Object) {
        super();
        for (let key in obj) {
            if (obj.hasOwnProperty(key)) {
                if (key == "fname") {
                    this.fname = obj["fname"];
                }
                if (key == "lname") {
                    this.lname = obj["lname"];
                }
            }
        }       

    }
}
var p = popupFactory(ConcretePopup, {"fname":"Niladri","lname":"D"});
console.log(p.fname); // Niladri
console.log(p.lname); // D

In this scenario, I am passing

popupType:  new (obj:Object) => T

as a type that requires an Object as a parameter in its constructor. Thus, we can supply the necessary properties and their corresponding values as an object through the data parameter. However, the drawback is having to manually extract properties for the derived class as demonstrated in the constructor of the ConcretePopup class above.

You can access a live example by clicking on this working fiddle link provided.

Utilizing generics within a function

Answer №1

Answer №2

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