Typescript doesn't check object property existence when it is typed as [key: string]

I have typed my object of objects as:

export interface ObjectInterface {
  [key: string]: SomeOtherObjectInterface;
}

Then let's say I have object like:

const obj: ObjectInterface = {
   a: ...,
   b: ...,
}

Then when I want to access some object property it is not type-safe:

const x = obj.dsdssdsdsds;

There is no error within my IDE.

When I remove type from obj it throws an error properly. What could be done here to use our type but still get errors like Property 'dsds' does not exist on type......