type Attributes = {
  [key: string]: number;
};

function Fn<KeysOfAttributes extends string>(opts: { attributes: Attributes }): any {
  // ...
}

// variant 1
const attributes = { // this object is hard coded (not dynamically generated)
  foo: 1,
  bar: 2,
  baz: 3
};

type Type = typeof attributes;
type Keys = keyof Type;

Fn<Keys>({
  attributes
});

// variant 2
const attributes: Attributes = {
  foo: 1,
  bar: 2,
  baz: 3
};

type Type = typeof attributes;// equals {[key: string]: number;}
type Keys = keyof Type;// equals string | number. Why ?

Fn<Keys>({// Here, I would like Keys to be "foo" | "bar" | "baz", instead I have string | number
  attributes
});

Type 'string | number' does not satisfy the constraint 'string'.
Type 'number' is not assignable to type 'string'.ts(2344)