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

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