Getting keys and value types of not instantiated Generic Type

Question

I'm creating a helper method to generate data for our tests. All I want is to be able to get the keys and types of the values of T so I can use it to create mocked values.

export class GenericMockFactory<T extends { id: number }>> {

makeMockModel(): T {
    return {
      //Mocked Data Here
    }
  }
}

So let's say T is

Model {
  id: number
  name: string
  country: string
}

And I have the mockFactory as

const mockFactory = GenericMockFactory<Model>

I'd like the return of mockFactory.makeMockModel() to be the Model object with values set as `any_${key}` if the type of the property is string and 1 if it's a number. For the example above:

{
  id: 1
  name: 'any_name'
  country: 'any_country'
}

What I've tried:

Object.entries(T).forEach(), which would be my go to if I had the object instantiated, I can't pass in a type, it seems.

This is similar to [this question](https://stackoverflow.com/questions/59480160/typescript-check-object-by-type-or-interface-at-runtime-with-typeguards-in-2020). The static type system is erased upon transpiling TS to JS, so the only way to do this is to make JS "schema" objects that represent TS types. You can do it manually, or use a custom TS transformer to do it programmatically. — jcalz, Nov 03 '20 at 19:44

Michel Vorwieger · Accepted Answer · 2020-11-03T19:39:26.013

0

This could help: https://stackoverflow.com/a/43572554/8350634

function mockObject(obj) {
    const newObj = {};
    const keys = Object.keys(obj);
    for(const key of keys) {
        if(typeof newObj[key] == "string") {
            newObj[key] = 'any' + key;
        } else {
        
            newObj[key] = 1;
        }
    }
    return newObj;
}

edited Nov 03 '20 at 19:39

answered Nov 03 '20 at 19:30

Michel Vorwieger

692
5
14

But then doesn't that require me to pass in an Object of type T? I wanted to just be able to pass T as type parameter and generate the object from that. Is there a way? – brightpants Nov 03 '20 at 19:31
1

Oh I see now what you mean. That would only be possible if the `type` has a `prototype` – Michel Vorwieger Nov 03 '20 at 19:32
So basically, only if it's instantiated somewhere else and then passed in, right? – brightpants Nov 03 '20 at 19:35
Yes, but you could also instantiate it in the function. If it doesn't have a constructor I guess. – Michel Vorwieger Nov 03 '20 at 19:37
1

Maybe this also helps you: https://stackoverflow.com/a/43572554/8350634 – Michel Vorwieger Nov 03 '20 at 19:38
I'm going home now but I'll check when I come back tomorrow, thanks for the time! – brightpants Nov 03 '20 at 19:59

Getting keys and value types of not instantiated Generic Type

1 Answers1