Create a package.json file

Start by creating a new directory wherever you keep your side projects in your local development environment. Inside that directory, use npm’s initializer command to create a package.json file:

mkdir node-express-typescript
cd node-express-typescript/
npm init --yes

When you initialize a package.json file, the --yes flag uses the default settings you have set up from npm config. The newly created package.json file might look something like the following code:

{
  "name": "express-typescript",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "type": "module",
  "keywords": [],
  "author": "",
  "license": "ISC"
}

Create a minimal server with Express

After initializing the package.json file, let’s add an Express package. From the terminal window, run the command below:

npm install express dotenv

Next, to trigger a minimal server, create a new file called server.js at the root of the project with the following code:

const express = require("express");
const dotenv = require("dotenv");

dotenv.config();

const app = express();
const port = process.env.PORT;

app.get("/", (req, res) => {
  res.send("Express + TypeScript Server");
});

app.listen(port, () => {
  console.log(`[server]: Server is running at https://localhost:${port}`);
});

The dotenv package is used to read environment variables from the .env file. Create it in the root of your Node.js app, then create the environment variable for PORT=8000.

To start the server, go back to the terminal and trigger the command node index.js:

The Express server is now up and running!

Installing TypeScript

We’ll start by installing TypeScript as a dev dependency. Along with it, we’ll install the the @types declaration packages for Express and Node.js, which provide type definitions in the form of declaration files.

Declaration files are predefined modules that describe the shape of JavaScript values, or the types present, for the TypeScript compiler. Type declarations are usually contained in files with a .d.ts extension. These declaration files are available for all libraries that were originally written in JavaScript, not TypeScript.

The DefinitelyTyped GitHub repository maintains the TypeScript type definitions for use directly in Node.js and other JavaScript projects, so you don’t have to define these types from scratch. To add these types or the declaration files related to a particular library or a module, you have to look for the packages that start with the @types namespace.

Open the terminal window and install the packages described above with the following command:

npm i -D typescript @types/express @types/node

The -D flag, also known as the --dev flag, is a specification for the package manager to install these libraries as devDependencies.

Once these libraries are installed, go to the package.json file where you’ll see a new devDependencies object:

{
  "devDependencies": {
    "@types/express": "^4.17.13",
    "@types/node": "^17.0.10",
    "typescript": "^4.5.5"
  }
}

Generating tsconfig.json

Now, we have a TypeScript project compiled with some default configuration options. The tsconfig.json file provides these default options and also provides us with the ability to tweak or customize the compiler options.

Typically, the tsconfig.json file lives at the root of the project. To generate it, we’ll use the tsc command:

npx tsc --init

The command above will generate a new file called tsconfig.json with the following default compiler options:

target: es2016 module: commonjs strict: true esModuleInterop: true skipLibCheck: true forceConsistentCasingInFileNames: true If you open the tsconfig.json file, you’ll see a lot of other compiler options that are commented out. In tsconfig.json, compilerOptions is a mandatory field that needs to be specified. The options used in the config above are:

target: Allows us to specify the target JavaScript version that the compiler will output module: Allows us to use a module manager in the compiled JavaScript code. CommonJS is supported and is a standard in Node.js strict: An option that enables strict type-checking options esModuleInterop: Allows us to compile ES6 modules to CommonJS modules skipLibCheck: If set to true, skips type-checking of default library declaration files forceConsistentCasingInFileNames: When set to true, enables case sensitive file naming One option that you will have to enable is called outDir, which specifies where the output will be located after the compilation step. You can search for this option in the tsconfig.json file and uncomment it.

By default, the value of this option is set to the root directory. Change it to dist:

{
  "compilerOptions": {
    "outDir": "./dist"

    // rest options remain same
  }
}

Create an Express server with a .ts extension

Now, you can easily convert the minimal server code in server.js to an server.ts file.

First, rename the file to server.ts. The .ts extension is a file extension that determines what TypeScript files are compiled to JavaScript files later when we build the server.

Open the server.ts file and modify it, as shown below:

import express, { Express, Request, Response } from "express";
import dotenv from "dotenv";

dotenv.config();

const app: Express = express();
const port = process.env.PORT;

app.get("/", (req: Request, res: Response) => {
  res.send("Express + TypeScript Server");
});

app.listen(port, () => {
  console.log(`⚡️[server]: Server is running at https://localhost:${port}`);
});

Watching file changes and build directory

Another development-related utility library I like to use when working on Node.js projects is nodemon. nodemon is a tool that helps develop Node.js based applications by automatically restarting the Node.js application when file changes in the directory are detected.

We’ll also install another dev dependency called Concurrently, which will allow us to run multiple commands like nodemon to watch file changes and the tsc command to compile the code:

npm install -D concurrently nodemon

After installing these dev dependencies, update the scripts in the package.json file:

{
  "scripts": {
    "build": "npx tsc",
    "start": "node dist/server.js",
    "dev": "concurrently \"npx tsc --watch\" \"nodemon -q dist/server.js\""
  }
}

The build command will compile the code in JavaScript inside a dist directory. The dev command is used to run the Node.js server in development mode.

Now, go back to the terminal window and run npm run dev to trigger the development server.