Typescript: type that has all the base attributes and allows any extensions

Question

Let's suppose I have an interface that describes only fields I'm interested in:

interface RestResponse {
    data: {
        name: string;
        value: number;
    }
    status: number;
}

I would like to use this interface as a type for a real REST response that has more fields than these above. E.g.:

const sampleResponse = {
    data: {
        name: 'cat',
        value: 64,
        description: 'the field I won't use anywhere'
    }
    status: 200,
    isSuccessful: true
}

The question is what type sampleResponse should be? I cannot make it:

const sampleResponse: RestResponse = { ... }

because the type doesn't have fields like isSuccessful and data.description.

It's an extension of RestResponse. I expect it to have all the fields specified in the interface, but don't mind having any additional ones.

---

So far I tried looking at https://www.typescriptlang.org/docs/handbook/advanced-types.html, but can't find anything useful for this case.

Answer 1

You can add an indexer, which allows any key which any value

That would look like this:

interface RestResponse {
  data: { 
    name: string;
    value: number;
  } 
  status: number;
  [key: string]: any;
}

What is a type indexer?

[key: string]: any is an object indexer.

[key: string] specifies that you can add any key to the object that is of type string

The : any after the indexer defines that the value of the key should be any and since any allows anything - you can add anything