Allow port 0 in next dev and next start

Co-authored-by: Steven <steven@ceriously.com>

packages/next/cli/next-dev.ts

@@ -75,9 +75,12 @@ const nextDev: cliCommand = (argv) => {
       )
     }
   }
-  const allowRetry = !args['--port']
-  let port: number =
-    args['--port'] || (process.env.PORT && parseInt(process.env.PORT)) || 3000
+
+  const hasPortFlag = Object.hasOwnProperty.call(args, '--port')
+  const allowRetry = !hasPortFlag
+  let port: number = hasPortFlag
+    ? args['--port']
+    : (process.env.PORT && parseInt(process.env.PORT)) || 3000
 
   // we allow the server to use a random port while testing
   // instead of attempting to find a random port and then hope

packages/next/cli/next-start.ts

@@ -52,8 +52,10 @@ const nextStart: cliCommand = (argv) => {
   }
 
   const dir = getProjectDir(args._[0])
-  let port: number =
-    args['--port'] || (process.env.PORT && parseInt(process.env.PORT)) || 3000
+  let port: number = Object.hasOwnProperty.call(args, '--port')
+    ? args['--port']
+    : (process.env.PORT && parseInt(process.env.PORT)) || 3000
+
   const host = args['--hostname'] || '0.0.0.0'
 
   if (process.env.__NEXT_FORCED_PORT) {

test/integration/cli/test/index.test.js

@@ -178,6 +178,19 @@ describe('CLI Usage', () => {
       expect(output).toMatch(new RegExp(`http://localhost:${port}`))
     })
 
+    test('--port 0', async () => {
+      const output = await runNextCommandDev([dir, '--port', '0'], true)
+      const matches = /on 0.0.0.0:(\d+)/.exec(output)
+      expect(matches).not.toBe(null)
+
+      const port = parseInt(matches[1])
+      // Regression test: port 0 was interpreted as if no port had been
+      // provided, falling back to 3000.
+      expect(port).not.toBe(3000)
+
+      expect(output).toMatch(new RegExp(`http://localhost:${port}`))
+    })
+
     test("NODE_OPTIONS='--inspect'", async () => {
       // this test checks that --inspect works by launching a single debugger for the main Next.js process,
       // not for its subprocesses