How to Set port in next.js

one application is running on port 3000 and I want to run another application on a different port of the default port. How I change this in React Next.js. My package.js script is

"scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "dev": "next",
    "build": "next build",
    "start": "next start"
  },

and start script command is npm run dev

This work for me

 "scripts": { 
       "dev": "next -p 8080" 
},

"scripts": {
    "dev": "next dev -p 8080", // for dev 
    "start": "next start -p 8080" // for prod
},

just need to do:

yarn dev -p PORT_YOU_LIKE

The application will start at http://localhost:3000 by default. The default port can be changed with -p, like so:

 npx next dev -p 4000

Or using the PORT environment variable:

PORT=4000 npx next dev

#note that I used npx not npm

You can also set the hostname to be different from the default of 0.0.0.0, this can be useful for making the application available for other devices on the network. The default hostname can be changed with -H, like so:

 npx next dev -H 192.168.1.2

If you're getting an error that the port is already in use, what you can do to resolve it on windows is

Go to the Task Manager.

Scroll and find a task process named. Node.js: Server-side

End this particular task.

There are two ways to do so:

In your package.json file, add -p 8080 to the dev/start scripts to start the server on port 8080:

  "scripts": {
    "dev": "next -p 8080",
    "build": "next build",
    "profile-build": "next build --profile",
    "start": "next start -p 8080"
  }

Alternatively, if you don't want to hardcode this in the package.json file, you could start the script with ENV variable PORT.

PORT=8080 npm run dev

Visit vercel documentation for more information.

Setting port number in npm script is not good idea at all.

From terminal you can pass the port number by using following command

SET PORT=3001 && npm start

A workaround using environment variables via .env file

Thanks to this github comment

For development

1. Create a script for your dev environment in the project root e.g. dev-server.js

// dev-server.js
require('dotenv').config(); // require dotenv
const cli = require('next/dist/cli/next-dev');

cli.nextDev(['-p', process.env.PORT || 3000]);

2. Then you can set a custom port in your .env like this: PORT=3002

3. Update the dev command in your package.json to use the dev-server.js script like this:

  "scripts": {
    "dev": "node dev-server.js"
  }

4. Run npm run dev and the NextJS application will start on port 3002.

For production

1. Create a script for your prod environment in the project root e.g. prod-server.js

// prod-server.js
require('dotenv').config(); // require dotenv
const cli = require('next/dist/cli/next-start');

cli.nextStart(['-p', process.env.PORT || 3000]);

2. Then you can set a custom port in your .env like this: PORT=3002

3. Update the start command in your package.json to use the prod-server.js script like this:

  "scripts": {
    "build": "next build",
    "start": "node prod-server.js"
  }

4. Run npm run start and the NextJS application will start on port 3002. (Don't forget to build the project before with npm run build)

dotenv should be installed via npm install dotenv, required and configured in the scripts as seen in the code snippets before.

Note from the github comment:

> There are some hosting providers that just force us to have server.js/index.js file. The bonus of the above solution is that it doesn't require any additional dependency.

With yarn you can easy pass any arguments:
yarn dev -p 8080 or yarn dev --port=8080.

With npm using -- to pass arguments:
npm run dev -- -p 8080