typedStruct

Typed wrapper around struct for keeping hand-written object guards in sync with an existing object type.

Optional keys in the target type must still be declared with optionalKey(...). This makes drift visible when the target type changes. This helper is not an OpenAPI validator or a schema generator.

import { isNumber, isString, optionalKey, typedStruct } from 'is-kit';

type User = {
  id: number;
  name: string;
  nickname?: string;
};

const isUser = typedStruct<User>()({
  id: isNumber,
  name: isString,
  nickname: optionalKey(isString),
});

isUser({ id: 1, name: 'Neko' }); // true
isUser({ id: 1, name: 'Neko', nickname: 'nya' }); // true
isUser({ id: 1, name: 'Neko', nickname: 123 }); // false

// typedStruct checks the hand-written struct fields against User.
typedStruct<User>()({
  id: isNumber,
  // TypeScript error: missing name and nickname fields
});

typedStruct<User>()({
  id: isNumber,
  name: isNumber,
  // TypeScript error: name must use a string guard
  nickname: optionalKey(isString),
});

// OpenAPI-generated types can be used the same way.
type PostResponse = {
  id: number;
  title: string;
  summary?: string;
};

const isPostResponse = typedStruct<PostResponse>()({
  id: isNumber,
  title: isString,
  summary: optionalKey(isString),
});