Building RESTful APIs with NestLib — Step-by-Step Tutorial

Migrating an Express App to NestLib: A Practical Walkthrough

Migrating an Express app to NestLib (a NestJS-based library/structure) helps you adopt a modular, opinionated framework that brings dependency injection, TypeScript-first architecture, and scalable patterns. This walkthrough shows a practical, step-by-step migration from an Express codebase to NestLib, assuming a medium-sized REST API with authentication, services, and a few routes.

1. Preparation and goals

• Goal: Move core HTTP API, middleware, services, and authentication from Express to NestLib with minimal regressions.
• Assumptions: Existing app uses Express with TypeScript (or will be converted), has route files, controllers, services, and middleware (e.g., logging, auth), and uses a database via an ORM (TypeORM/Prisma/Sequelize).
• Checklist before starting:
  1. Add TypeScript if not present and confirm tsconfig is compatible.
  2. Run tests and create a baseline branch.
  3. Note environment variables and config structure.
  4. Install Node 14+ (prefer 16+) and update dependencies.

2. Create a new NestLib project scaffold

1. Install Nest CLI and generate a new project:

   • npm i -g @nestjs/cli nest new nestlib-app 

2. Choose the package manager and initial options. This scaffold includes main.ts, AppModule, a sample controller, and basic structure.
3. Verify the dev server:

   • npm run start:dev 

3. Project structure mapping

Map your Express folders to NestLib modules:

• Express route files -> Nest controllers (per domain)
• Express