Description

This is a small example about how to add NestJS to existing legacy app running on Express.

How to integrate Nest.JS to Express app:

• Install @nestjs/cli:

$ npm i -g @nestjs/cli

• Rename your original package.json, e.g. package-old.json
• [Optional] Rename your old src folder if exist, e.g. src-legacy
• Create new NestJS project using this command inside your root folder using command:

$ nest new . -g -s -p npm -l TS

• Copy required dependencies from your legacy app (package-old.json) into the new package.json
• Install node modules

$ npm i

• Modify start commands in package.json file to run your file with an old express entrypoint, e.g. app.js:

"start": "npx ts-node app.js",
"start:prod": "node dist/app.js",

• [Optional] If your legacy project contains js code, allow it inside the compilerOptions in tsconfig.json:

"allowJs": true,

• Modify src/main.ts:

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';
import { ExpressAdapter } from '@nestjs/platform-express';

export async function bootstrap(express) {
  const adapter = new ExpressAdapter(express);
  const app = await NestFactory.create(AppModule, adapter);

  await app.init();

  return app;
}

• Modify src/app.controller.ts to test NestJS endpoints later, e.g.:

@Controller('v2')

• Pass your existing express into newly created Nest bootstrap function. In this example it's inside app.js file:

import * as express from 'express';
import { getHello } from './src-legacy/my-legacy-service.js';
import { bootstrap } from './src/main';

const app = express();
const port = 3000;

app.get('/v1', (req, res) => {
  res.send(getHello());
});

bootstrap(app).then((nestApp) => {
  nestApp.listen(port, () => {
    console.log(`app running on port ${port}`);
  });
});

• Build and run app:

# development
$ npm run start

# production mode
$ npm run build
$ npm run start:prod

• Verify your old and new NestJS endpoints

How to run this example

Installation

$ npm i

Running the app

# development
$ npm run start

# production mode
$ npm run build
$ npm run start:prod

Nest JS Docs: https://docs.nestjs.com/