pick-keys

PickKeys<Type, Value> constructs a union type by picking all properties of object type Type which values are assignable to type Value

interface StudentWithOptionalName {
  homework: string | undefined;
  name?: string;
  score: number;
}

type NumericStudentProperties = PickKeys<StudentWithOptionalName, number>;
//   ^? 'score'

Optional properties are treated as | undefined and are included in this case (e.g. property name)

type MaybeStringStudentProperties = PickKeys<StudentWithOptionalName, string | undefined>;
//   ^? 'homework' | 'name'

It includes property which values are equal to Value or narrower than Value

type AllStudentProperties = PickKeys<StudentWithOptionalName, number | string | undefined>;
//   ^? 'homework' | 'name' | 'score'

When there is no property of value equal to Value ot narrower than Value, it returns never

type NoStudentProperties = PickKeys<StudentWithOptionalName, string>;
//   ^? never

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