TypeScript: Infer literal type of already defined type

Question

Not sure if this is possible or to best describe this, but I need to infer the literal type of a type that's already defined by another type/interface.

For example, I've got an object that adheres to a certain interface:

interface MyObject {
    foo: string | number;
    bar: string[];
}
const MY_OBJECT: MyObject = {
    foo: 123,
    bar: ["abc", "def"],
};

Now, what I'd like to know is the type literal of MY_OBJECT, by doing something like this:

type MyObjectLiteral = Infer<typeof MY_OBJECT>;

That should make MyObjectLiteral: { foo: 123, bar: ["abc", "def"] }

The question is how to define type Infer<T> to return the inferred type of T.

Any help or clarification would be appreciated.

Answer 1

This isn't really possible with types alone. A soon as you assign a value to a type that is wider than what could be inferred, you lose that type information.

But you can always treat a more narrow type as the wider type that includes it. Which means you can always go the other direction.

---

You could simply declare your object as const and then use it anywhere that accepts a MyObject type.

interface MyObject {
    foo: string | number;
    bar: readonly string[];
}

const myObject = { foo: 123, bar: ["abc", "def"] } as const
const barZero = myObject.bar[0] // type: "abc"

const myObjectWide: MyObject = myObject // works
const barWideZero = myObjectWide.bar[0] // type: string

Playground

---

Or if you want to constrain the construction of myObject you can use a function to capture the exact subtype, and return it.

For example:

interface MyObject {
    foo: string | number;
    bar: readonly string[];
}

function makeObj<T extends MyObject>(myObject: T): T {
    return myObject
}

const myBadObject = makeObj({ bad: true } as const)
// Argument of type '{ readonly bad: true; }' is not assignable to parameter of type 'MyObject'.
//  Object literal may only specify known properties, and 'bad' does not exist in type 'MyObject'.(2345)

const myObject = makeObj({ foo: 123, bar: ["abc", "def"] } as const)
const barZero = myObject.bar[0] // type: "abc"

const myObjectWide: MyObject = myObject // works
const barWideZero = myObjectWide.bar[0] // type: string

Playground