add pretty message if port already in use(#927)

bin/next-dev

@@ -2,7 +2,7 @@
 import 'source-map-support/register'
 import { resolve, join } from 'path'
 import parseArgs from 'minimist'
-import { existsSync } from 'fs'
+import { existsSync, readFileSync } from 'fs'
 import Server from '../server'
 import { printAndExit } from '../lib/utils'
 
@@ -61,6 +61,19 @@ srv.start(argv.port)
   }
 })
 .catch((err) => {
-  console.error(err)
-  process.exit(1)
+  if (err.code === 'EADDRINUSE') {
+    let errorMessage = `Port ${argv.port} is already in use.`
+    if (existsSync(join(dir, 'package.json'))) {
+      try {
+        const appPackage = JSON.parse(readFileSync(join(dir, 'package.json'), 'utf8'))
+        const nextTuple = Object.entries(appPackage.scripts).find(i => i[1] === 'next')
+        errorMessage += `\nUse \`npm run ${nextTuple[0]} -- -p <some other port>\`.`
+      } catch (e) { }
+    }
+
+    console.error(errorMessage)
+  } else {
+    console.error(err)
+  }
+  process.nextTick(() => process.exit(1))
 })

server/index.js

@@ -119,10 +119,9 @@ export default class Server {
     await this.prepare()
     this.http = http.createServer(this.getRequestHandler())
     await new Promise((resolve, reject) => {
-      this.http.listen(port, (err) => {
-        if (err) return reject(err)
-        resolve()
-      })
+      this.http.on('error', reject)
+      this.http.on('listening', () => resolve())
+      this.http.listen(port, () => {})
     })
   }