assertExactKeys.ts

assertExactKeys.ts

/**
 * ✅ Use case:
 * Validate that a list of string keys matches a TypeScript interface *exactly*:
 * - No missing keys
 * - No extra keys
 *
 * 💡 Great for:
 * - DTO serialization (`toDBObject`)
 * - Form field definitions
 * - Schema-driven rendering
 * - Backend API constraints
 *
 * ⛔ Catches bugs at compile-time — safe refactors guaranteed.
 *
 * Example below ensures `documentKeys` matches all keys of `IDocument` exactly.
 */

export function assertExactKeys<T>() {
  return function <K extends readonly (keyof T)[]>(
    keys: K &
      (Exclude<keyof T, K[number]> extends never
        ? Exclude<K[number], keyof T> extends never
          ? unknown
          : '❌ Extra keys in key list'
        : '❌ Missing keys in key list')
  ) {
    return keys;
  };
}

// --- Example usage ---

export interface IDocument {
  id: string;
  name: string;
  baseColor?: string;
  baseColorHist?: string[];
  createdAt: number;
  updatedAt: number;
}

// ✅ Matches IDocument keys exactly — will error if one is missing or extra
export const documentKeys = assertExactKeys<IDocument>()([
  'id',
  'name',
  'baseColor',
  'baseColorHist',
  'createdAt',
  'updatedAt',
] as const);