TypeScriptuser-service.ts
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interface User {
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  id: string;
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  name: string;
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  email: string;
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  password: string;
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  createdAt: Date;
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  role: 'admin' | 'user' | 'guest';
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}
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// Derived types — no duplication
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type CreateUserDto = Omit<User, 'id' | 'createdAt'>;
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type UpdateUserDto = Partial<Omit<User, 'id' | 'createdAt'>>;
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type UserPublic = Omit<User, 'password'>;
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type UserPreview = Pick<User, 'id' | 'name' | 'role'>;
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type RolePermissions = Record<User['role'], string[]>;
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const permissions: RolePermissions = {
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  admin: ['read', 'write', 'delete', 'manage'],
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  user: ['read', 'write'],
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  guest: ['read'],
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};
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// Simulated service
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class UserService {
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  private users = new Map<string, User>();
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  create(dto: CreateUserDto): UserPublic {
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    const user: User = { ...dto, id: crypto.randomUUID(), createdAt: new Date() };
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    this.users.set(user.id, user);
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    const { password, ...publicUser } = user;
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    return publicUser;
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  }
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  update(id: string, dto: UpdateUserDto): UserPublic | null {
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    const user = this.users.get(id);
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    if (!user) return null;
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    const updated = { ...user, ...dto };
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    this.users.set(id, updated);
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    const { password, ...publicUser } = updated;
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    return publicUser;
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  }
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  list(): UserPreview[] {
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    return Array.from(this.users.values()).map(({ id, name, role }) => ({ id, name, role }));
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  }
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}
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const svc = new UserService();
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const user = svc.create({ name: 'Alice', email: 'alice@x.com', password: 'secret', role: 'user' });
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const updated = svc.update(user.id, { name: 'Alice Smith' }); // Only name changes
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console.log(svc.list());