Get keys from object which uses an interface in typescript

Question

Can I get an objects actual keys when I'm using an interface to describe the object?

Example below

interface IPerson {
    name: string;
}
interface IAddress {
    [key: string]: IPerson;
}

const personInAddressObj: IAddress= {
    someAddress1: {
        name: 'John',
    },
    someAddress2: {
        name: 'Jacob',
    }
} as const

type Keys = keyof typeof personInAddressObj;

Id want the type Keys to have the values "someAddress1 | someAddress2". If I take the interface out of "personInAddressObj", I can get the keys. However, when the interface is used, I can't get the actual keys out of the object.

If you mean using those keys as type (that what's you write), then you can't. Typescript compiles on static info, hence you can't have runtime data at compile-time. However, if the keys are used for runtime stuffs, you can leverage the Object.keys(personInAddressObj), which gets you the keys as array of strings. Have a look here: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — Mario Vernari
– Mario Vernari, Commented Dec 9, 2022 at 7:24
See here for the corresponding open TypeScript feature request. — Robby Cornelissen
– Robby Cornelissen, Commented Dec 9, 2022 at 7:39
@MarioVernari Objects declared with a const assertion (as const) are compile-time information. — Robby Cornelissen
– Robby Cornelissen, Commented Dec 9, 2022 at 7:41
"If I take the interface out of personInAddressObj, I can get the keys." - so, why not do that? There's no point in using as const if you did assign to a variable declared as IAddress. — Bergi
– Bergi, Commented Dec 9, 2022 at 7:45
@RobbyCornelissen that's new to me. Many thanks for pointing me out. — Mario Vernari
– Mario Vernari, Commented Dec 9, 2022 at 7:54

Robby Cornelissen · Accepted Answer · 2022-12-09 08:01:32Z

4

There's an open feature request for this specific use case.

For your specific example, I think you can use the new (since TypeScript 4.9) satisfies operator to have both a const assertion and type checking:

interface IPerson {
    name: string;
}

interface IAddress {
    [key: string]: IPerson;
}

const personInAddressObj = {
    someAddress1: {
        name: 'John'
    },
    someAddress2: {
        name: 'Jacob'
    }
} as const satisfies IAddress;

type Keys = keyof typeof personInAddressObj;
// ↑ inferred as "someAddress1" | "someAddress2"

Playground link

edited Dec 9, 2022 at 8:01

answered Dec 9, 2022 at 7:49

Robby Cornelissen

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1 Comment

Kustemit Over a year ago

This is exactly what I wanted to achieve. Thanks for helping out!

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