next.js/packages/next/bin/next-dev

#!/usr/bin/env node
import { resolve, join } from 'path'
import parseArgs from 'minimist'
import { existsSync } from 'fs'
import startServer from '../server/lib/start-server'
import { printAndExit } from '../server/lib/utils'

const argv = parseArgs(process.argv.slice(2), {
  alias: {
    h: 'help',
    H: 'hostname',
    p: 'port'
  },
  boolean: ['h'],
  string: ['H'],
  default: { p: 3000 }
})

if (argv.help) {
  console.log(`
    Description
      Starts the application in development mode (hot-code reloading, error
      reporting, etc)

    Usage
      $ next dev <dir> -p <port number>

    <dir> represents where the compiled folder should go.
    If no directory is provided, the folder will be created in the current directory.
    You can set a custom folder in config https://github.com/zeit/next.js#custom-configuration.

    Options
      --port, -p      A port number on which to start the application
      --hostname, -H  Hostname on which to start the application
      --help, -h      Displays this message
  `)
  process.exit(0)
}

const dir = resolve(argv._[0] || '.')

// Check if pages dir exists and warn if not
if (!existsSync(dir)) {
  printAndExit(`> No such directory exists as the project root: ${dir}`)
}

if (!existsSync(join(dir, 'pages'))) {
  if (existsSync(join(dir, '..', 'pages'))) {
    printAndExit('> No `pages` directory found. Did you mean to run `next` in the parent (`../`) directory?')
  }

  printAndExit('> Couldn\'t find a `pages` directory. Please create one under the project root')
}

startServer({dir, dev: true}, argv.port, argv.hostname)
  .then(async () => {
    console.log(`> Ready on http://${argv.hostname ? argv.hostname : 'localhost'}:${argv.port}`)
  })
  .catch((err) => {
    if (err.code === 'EADDRINUSE') {
      let errorMessage = `Port ${argv.port} is already in use.`
      const pkgAppPath = require('find-up').sync('package.json', {
        cwd: dir
      })
      const appPackage = require(pkgAppPath)
      if (appPackage.scripts) {
        const nextScript = Object.entries(appPackage.scripts).find(scriptLine => scriptLine[1] === 'next')
        if (nextScript) {
          errorMessage += `\nUse \`npm run ${nextScript[0]} -- -p <some other port>\`.`
        }
      }

      console.error(errorMessage)
    } else {
      console.error(err)
    }
    process.nextTick(() => process.exit(1))
  })