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Express.js with TypeScript

Using TypeScript with Express.js brings static typing to your backend application, improving code quality, autocomplete in editors, and early error detection during development.

Setup and Installation

First, initialize a new Node.js project and install the necessary dependencies:

Terminal window
npm init -y
npm install express
npm install -D typescript @types/node @types/express ts-node
  • typescript: The TypeScript compiler.
  • @types/node & @types/express: Type definitions for Node.js and Express.
  • ts-node: An execution environment that runs TypeScript directly.

Configuring TypeScript

Initialize a tsconfig.json file in your project root:

Terminal window
npx tsc --init

Update your tsconfig.json with recommended settings for Express:

{
  "compilerOptions": {
    "target": "es2022",
    "module": "commonjs",
    "rootDir": "./src",
    "outDir": "./dist",
    "strict": true,
    "esModuleInterop": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true
  }
}

Creating a Basic Server

Create a src directory and add an index.ts file:

import express, { Request, Response, Application } from 'express';


const app: Application = express();
const PORT = process.env.PORT || 3000;


app.use(express.json());


app.get('/', (req: Request, res: Response) => {
  res.send('Hello from Express with TypeScript!');
});


app.listen(PORT, () => {
  console.log(`Server is running on port ${PORT}`);
});

Notice how we use the Request, Response, and Application types provided by @types/express to strongly type our application.

Running the Application

You can use ts-node to run the application during development. Add the following scripts to your package.json:

"scripts": {
  "start": "node dist/index.js",
  "dev": "ts-node src/index.ts",
  "build": "tsc"
}

Now you can run the development server:

Terminal window
npm run dev

For production, you should build the project using npm run build and then run the compiled JavaScript file.

Extending the Request Object

A common pattern in Express is attaching custom properties to the Request object (e.g., user data from authentication middleware). To do this in TypeScript, you need to use Declaration Merging:

Create a types/express/index.d.ts file:

import { User } from '../../models/User';


declare global {
  namespace Express {
    interface Request {
      user?: User; // Assuming you have a User model or interface
    }
  }
}

This allows you to safely access req.user in your route handlers without TypeScript throwing errors.