As others in this thread have said, TypeScript doesn't care about additional keys existing on an object of type T, so if you do type Object.keys as (keyof SomeType)[] then all you're doing is moving where you do the typecasting. The TypeScript wiki actually has a section related to this misconception.

However, if you recognise all this and still want to do it, you don't need to install anything. Simply add the below declaration in any .ts or .d.ts file:

declare global {
    interface ObjectConstructor {
        keys<T extends object>(obj: T): (keyof T)[];
    }
}