Comment by skywal_l

    type Ensure

I define a type called Ensure

    <T, K extends keyof T>

This type takes two type parameters, one called T and the other K which will consist of Keys belonging to the type T (in our case, "foo", "bar" or "baz").

    = T & 

This new type (called Ensure) will be equal to the union of two types: One will be T and the other will be:

    { [U in keyof Pick<T, K>]

A new type which keys will be picked among the key listed in K

    -?

To which we will remove the potential optional qualifier

    : T[U] };

And which types will be the same as in T.