TypeScript: an interface property dependent on another

Question

is it possible to type an interface property dependent on another?

For example I have:

const object = {
  foo: 'hello',
  bar: { hello: '123', },
}

And I want to make sure that the key of bar must be the value of foo.

interface ObjectType = {
  foo: string;
  bar: { hello: string; } // instead of hardcoding have something like this? --> payload: { ValueOfFoo: string; }
}

Thanks! :)

Answer 1

You need a generic to capture the type of a specific string

interface ObjectType<K extends string> {
    foo: K;
    bar: { [P in K]: string };
}

Then you can write

const object: ObjectType<'hello'> = {
    foo: 'hello',
    bar: { hello: '123', },
};

Or, if you have a function that takes such an object you can use the interface to enforce the rule with full type inference

function useObject<K extends string>(o: ObjectType<K>) { }

useObject({
    foo: 'hello', 
    bar: { hello: '113' } // works
});

useObject({
    foo: 'hello', 
    bar: { world: '113' }
//         ^^^^^−−−−−−−−−−−− error, `world` should be `hello`
});

Playground link