function hasKey<O>(obj: O, key: PropertyKey): key is keyof O { return key in obj }

If there is no relationship between the value's type and the type in the type predicate, the guard would make no sense. For example, TypeScript won't allow a user-defined guard like this: function isString(value: Date): value is string { return typeof value === "string"; }

Two of the predominant types of relationships in knowledge-representation systems are predication and the universally quantified conditional.

It is based on the idea that each validation is encapsulated by a simple, stateless predicate that receives some input and returns either true or false.

the function you pass to the find method is called a predicate. The predicate here defines a boolean outcome based on conditions defined in the function itself, so that the find method can determine which value to find.