Creating Instances of Parameterized Types

Question

Creating Instances of Parameterized Types

Consider the following scenario:

class Datum {}

An error message (

error TS2304: Cannot find name 'T'

) is encountered when attempting the following:

class Data<T extends Datum> {
    datum: T

    constructor() {
        this.datum = new T()
    }
}

Subsequently, an attempt is made which results in another error message (

Type 'Datum' is not assignable to type 'T'

):

 class Data<T extends Datum> {
    datum: T

    constructor() {
        this.datum = new Datum();
    }
}

Query: Is it impossible to create instances of the constrained types like T? The expectation was that since T is restricted to extend Datum, assigning datum: T = new Datum() should be feasible.

generics typescript instantiation

Answer 1

Answer №1

Don't forget two important things: First off, generics are completely removed during compilation. They don't have any impact at runtime, so trying to treat a generic type as if it were a runtime value is pointless.

Secondly, it's worth noting that a subclass of Datum may have constructor parameters. Even if T actually existed, you can't just blindly call new on it with no arguments.

Putting it all together, what you need is something like this:

class Datum {}

class Data<T extends Datum> {
    datum: T

    constructor(ctor: new() => T) {
        this.datum = new ctor();
    }
}

class ByteDatum extends Datum {
    new() { }
}

let y = new Data(ByteDatum);
let x = y.datum; // x: ByteDatum

Answer 2

Don't forget two important things: First off, generics are completely removed during compilation. They don't have any impact at runtime, so trying to treat a generic type as if it were a runtime value is pointless.

Secondly, it's worth noting that a subclass of Datum may have constructor parameters. Even if T actually existed, you can't just blindly call new on it with no arguments.

Putting it all together, what you need is something like this:

class Datum {}

class Data<T extends Datum> {
    datum: T

    constructor(ctor: new() => T) {
        this.datum = new ctor();
    }
}

class ByteDatum extends Datum {
    new() { }
}

let y = new Data(ByteDatum);
let x = y.datum; // x: ByteDatum

Creating Instances of Parameterized Types

Answer №1

Similar questions

The functionality of verifying the type of an item in a list using Typescript is not functioning

Angular 5: Issues with retrieving response using HttpClient's get request

Speedy Typescript inquiry query

What is the best way to display a Nested JSON structure without an object key?

Having difficulty importing the WebRTCAdaptor from the antmedia package stored in the node modules directory into an Angular Typescript file

What is the proper way to call document methods, like getElementByID, in a tsx file?

Send a collection of objects by submitting a form

"Exploring Angular: A guide to scrolling to the bottom of a page with

Angular2 error: "missing exported member 'bootstrap'"

The process of automatically formatting Typescript has transformed into an unfortunate auto-discarding action

Why do variables in an HTML file fail to update after being navigated within onAuthStateChanged?

The element you are trying to access, "noUiSlider," does not belong to the type "HTMLElement" and cannot be found

Is there a way to dynamically adjust the size of mat dialog content to match the size of the mat dialog in

Using AngularFire2 to manage your data services?

What could be causing the TypeScript exhaustive switch check to malfunction?

Looking to update the state of a nested object with useReducer?

Creating a dynamic image binding feature in Angular 8

The type 'Requireable<string>' cannot be matched with the type 'Validator<"horizontal" | "vertical" | undefined>'

Deciphering intricate Type Script Type declarations

When evaluating code with eval, properties of undefined cannot be set, but the process works seamlessly without