strict-deep-pick

StrictDeepPick<Type, Filter> constructs a type by picking set of properties, which have property values never or true in type Filter, from type Type

interface EnglishClass {
  students: { name: string; score: number }[];
  teacher: {
    name: string;
    yearsOfExperience: number;
  };
  year: number;
}

type EnglishClassNamesOnly = StrictDeepPick<
  // ^? { students: { name: string }[]; teacher: { name: string }; }
  EnglishClass,
  {
    students: { name: never }[];
    teacher: {
      name: never;
    };
  }
>;

It validates that all Filter properties exist in type Type

// error: Type '{ id: never; students: { id: never; }[]; teacher: { id: never; }; }' does not satisfy the constraint '{ students?: true | DeepModify<{ name: string; score: number; }[]> | undefined; teacher?: true | { name?: true | undefined; yearsOfExperience?: true | undefined; } | undefined; year?: true | undefined; }'.
//   Types of property 'students' are incompatible.
//     Type '{ id: never; }[]' is not assignable to type 'true | DeepModify<{ name: string; score: number; }[]> | undefined'.
//       Type '{ id: never; }[]' is not assignable to type '(true | { name?: true | undefined; score?: true | undefined; } | undefined)[]'.
//         Type '{ id: never; }' is not assignable to type 'true | { name?: true | undefined; score?: true | undefined; } | undefined'.
type UnknownEnglishClass = StrictDeepPick<
  EnglishClass,
  {
    id: never;
    students: { id: never }[];
    teacher: {
      id: never;
    };
  }
  // ^^^^^^^^^^^^^^^^^^^^^^^^
>;

If you don't need a second type parameter Filter to be validated against a structure of a first type parameter Type, please use DeepPick<Type, Filter>.

Useful in functions which cannot access specified properties

declare const englishClass: EnglishClass;

const calculateAverageScore = (englishClass: EnglishClassNamesOnly) => {
  let scoreSum = 0;

  for (const student of englishClass.students) {
    // Property 'score' does not exist on type '{ name: string; }'
    scoreSum += student.score;
    //                  ^^^^^
  }

  return scoreSum / englishClass.students.length;
};

calculateAverageScore(englishClass);

⚠️ Limitations:

• StrictDeepPick cannot be used when Type is generic type – #343 (please use DeepPick instead)
• (bug) StrictDeepPick cannot be used on nullable type - #380
• StrictDeepPick only limits access to specified properties in your codebase, but doesn't remove them in runtime

TS Playground – https://tsplay.dev/w1GJkm