What is a Module

Property	Meaning
`controllers`	Controllers of this module (NestJS creates them and maps their routes)
`providers`	Services and other injectables created in this module
`imports`	Other modules whose exported providers this module needs
`exports`	Providers from this module that importing modules may use

Example

NestJS walks the module tree from the root and registers every module once, even if it is imported many times. This plain TypeScript version shows that walk:

interface ModuleDef {
  name: string;
  imports: ModuleDef[];
  providers: string[];
}

const DatabaseModule: ModuleDef = { name: 'DatabaseModule', imports: [], providers: ['DataSource'] };
const UsersModule: ModuleDef = { name: 'UsersModule', imports: [DatabaseModule], providers: ['UsersService'] };
const ProductsModule: ModuleDef = { name: 'ProductsModule', imports: [DatabaseModule], providers: ['ProductsService'] };
const OrdersModule: ModuleDef = { name: 'OrdersModule', imports: [DatabaseModule, UsersModule, ProductsModule], providers: ['OrdersService'] };
const AppModule: ModuleDef = { name: 'AppModule', imports: [ProductsModule, OrdersModule], providers: [] };

const seen = new Set<ModuleDef>();
function scan(mod: ModuleDef, depth: number): void {
  if (seen.has(mod)) {
    console.log('  '.repeat(depth) + mod.name + ' (already loaded)');
    return;
  }
  seen.add(mod);
  console.log('  '.repeat(depth) + mod.name + ' [' + mod.providers.join(', ') + ']');
  for (const child of mod.imports) scan(child, depth + 1);
}

scan(AppModule, 0);
console.log('modules initialized:', seen.size);

Output

AppModule []
  ProductsModule [ProductsService]
    DatabaseModule [DataSource]
  OrdersModule [OrdersService]
    DatabaseModule (already loaded)
    UsersModule [UsersService]
      DatabaseModule (already loaded)
    ProductsModule (already loaded)
modules initialized: 5

The @Module Decorator

How NestJS Reads Modules

Example