Node.js npm

npm is the package manager for Node.js. It installs code that other developers have shared, so you do not have to write everything yourself.

What is npm?

• npm stands for Node Package Manager
• It is installed together with Node.js
• The npm registry has millions of free packages (libraries and tools)
• It also runs your project's scripts, like starting a server or running tests

Creating package.json

Every Node.js project has a package.json file. It stores the project's name, its scripts and the packages it needs. Create one with:

npm init -y

{
  "name": "school-api",
  "version": "1.0.0",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "license": "ISC"
}

-y accepts all the default answers. You can edit the file later.

Installing a Package

npm install dayjs

Three things happen:

1. The package is downloaded into a node_modules folder
2. It is added to dependencies in package.json
3. Exact versions are saved in package-lock.json

Now use it:

import dayjs from "dayjs";

const admission = dayjs("2026-10-15");
console.log("Admission:", admission.format("DD MMM YYYY"));
console.log("Course ends:", admission.add(6, "month").format("DD MMM YYYY"));

Admission: 15 Oct 2026
Course ends: 15 Apr 2027

(This uses import, so add "type": "module" to package.json as shown in the Modules lesson.)

Common npm Commands

dependencies and devDependencies

• dependencies are needed to run the app, such as express
• devDependencies are only needed while developing, such as testing or linting tools

"dependencies": {
  "express": "^5.1.0"
},
"devDependencies": {
  "eslint": "^9.30.0"
}

Version Numbers

Versions follow semantic versioning: MAJOR.MINOR.PATCH, such as 5.1.0.

The ^ in ^5.1.0 means "any 5.x version from 5.1.0 up". The ~ in ~5.1.0 means "only 5.1.x patches".

Scripts

Add your own commands to the scripts section:

"scripts": {
  "start": "node index.js",
  "dev": "node --watch index.js"
}

npm run dev
npm start

start and test are special: you can run them without run.

node_modules and Git

The node_modules folder can be very large. Never upload it to GitHub or send it to a client. Add it to .gitignore:

node_modules
.env

Anyone who downloads your project runs npm install to get the same packages, using package-lock.json for the exact versions.

Exercise