Typescript: Extending from {}

Question

Given:

class A<T extends {
  [k: string]: any
}> {
  private model: T

  constructor(model: T = {}) {
    this.model = model
  }
}

Why do i got an error stating

Type '{}' is not assignable to type 'T'.

Doesn't {} is assignable to

{ [k: string]: any }

Thank you.

This feels like a bug. Did you check the typescript issues list? — Evert
– Evert, Commented Apr 27, 2019 at 4:01

Ryan Cavanaugh · Accepted Answer · 2019-04-27 04:28:35Z

1

This is a correct error.

Your class says that this code is legal:

type MyType = {
    a: string;
    b: number;
}

const c = new A<MyType>();

But now you have an A whose model value does not have the required a and b properties.

answered Apr 27, 2019 at 4:28

Ryan Cavanaugh

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Comments

Fyodor Yemelyanenko · Accepted Answer · 2019-04-28 09:28:06Z

0

This issue is described here. The solution to issue is as below

class A<T extends {
    [k: string]: any
}> {
    private model: T

    constructor(model: T = <T>{}) {
        this.model = model
    }
}

Using type cast in constructor is essential model: T = <T>{}

Then you cn instantiate class A with just

var instA = new A();
// instA.model will be {}

You may also read this It has links and discussions about similar issues.

edited Apr 28, 2019 at 9:28

answered Apr 27, 2019 at 5:36

Fyodor Yemelyanenko

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2 Comments

Andree Christian Over a year ago

Yes, i noticed this when trying to solve it. But i think it shouldn't be the answer because why would {} isn't assignable to {[k: string]?: any} in the first place.

Fyodor Yemelyanenko Over a year ago

I've made edit with lnks to github issue about this case

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