Typescript create object of a type which comes from a variable

Question

A method returns me a class type: Widget.

I want to create a object of this type with the following code:

const wType = def.getWidgetType(); // returns Widget as type
const obj = new wType('foo'); // use the const like a normal type with parameters

getWidgetType()

public getWidgetType(): any {
 return TextWidget;
}

Error

error TS2351: Cannot use 'new' with an expression whose type lacks a call or construct signature.

Is there a "nice" version (without eval) to create a object with a given class type?

What is the signature of getWidgetType ?

Titian Cernicova-Dragomir
– Titian Cernicova-Dragomir

2018-06-05 06:32:26 +00:00
Commented Jun 5, 2018 at 6:32 — Titian Cernicova-Dragomir
– Titian Cernicova-Dragomir, Commented Jun 5, 2018 at 6:32
Show us your getWidgetType

Isaac
– Isaac

2018-06-05 06:33:27 +00:00
Commented Jun 5, 2018 at 6:33 — Isaac
– Isaac, Commented Jun 5, 2018 at 6:33

Titian Cernicova-Dragomir · Accepted Answer · 2018-06-05 06:37:42Z

5

Assuming what getWidgetType returns is a constructor, you can invoke new wType('foo') provided the signature of getWidgetType explicitly states it returns a constructor signature.

For example this code would be valid:

class Definition<T> {
    // Takes in a constructor
    constructor(public ctor: new (p: string) => T) {

    }
    // returns a constructor (aka a function that can be used with the new operator)
    // return type annotation could be inferred here, was added for demonstrative purposes 
    getWidgetType() : new (p: string) => T{
        return this.ctor;
    }
}

const def = new Definition(class {
    constructor(p: string) {}
});

const wType = def.getWidgetType();
const obj = new wType('foo')

answered Jun 5, 2018 at 6:37

Titian Cernicova-Dragomir

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1 Comment

Joba Over a year ago

Your example helped me to fix my stuff. Thanks! In my code I just had to change the return type to getWidgetType(): new (id: string) => TextWidget

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