It feels amazing when you just type 'npm start' and your app is up and running. But what’s actually happening under the hood?
Whether you’re using React, Node.js, or Next.js, this one simple command kicks off a powerful chain reaction. In this blog, let’s break it all down from the very beginning.

WHAT IS NPM?

npm stands for Node Package Manager. It does two things:
Installs and manages packages
E.g., npm install express adds Express to your project.
npm install mongoose adds mongoose to your project.
It also runs custom scripts defined in your project’s package.json
E.g., npm start, npm test, npm run build

But why package.json?
package.json is the heart of your Node.js project. It contains:

• Project metadata (name, version, etc.)
• List of dependencies and devDependencies
• Scripts section → where npm start is defined!

{
  "name": "my-app",
  "version": "1.0.0",
  "scripts": {
    "start": "node index.js"
  },
  "dependencies": {
    "express": "^4.18.2"
  }
}

When you type npm start, npm first checks your package.json file for a scripts section. If a "start" script is defined then npm will execute the command associated with that script — in this case, node index.js. If it doesn’t exist, npm throws an error: npm ERR! Missing script: "start"

After finding the start script, npm runs the command in your terminal eg node index.js, react-scripts start, next dev, etc. is executed in your terminal as a child process.
You could define anything here — even:
"start": "echo Hello, Developers!"
Try running that and watch what happens.

Before your app starts, some environment prep might happen:

• process.env.NODE_ENV is typically set to development or production
• .env files may be loaded using dotenv or framework internals
• Preload scripts like babel-register or ts-node/register might run (in dev tools)

What Happens Under the Hood in Modern Frameworks?

Whether it’s react-scripts, next, or vite, these tools often:

• Watches file changes with tools like chokidar
• Compiles JS/TS with Babel, SWC, or esbuild
• Bundles code with Webpack, Rollup, or Vite
• Serves your app locally via a dev server
• Enables hot reload or fast refresh
• Generates source maps for debugging All of this is triggered by that single "start" line in your package.json

What happens next depends on the type of project
So next time you type npm start, just remember — you’re launching a whole system that transforms your source code into a live, working app!