fix(server): allow listening on number 0 port (#4781)

packages/server/src/server.js

@@ -225,7 +225,7 @@ export default class Server {
   async listen(port, host, socket) {
     // Create a new listener
     const listener = new Listener({
-      port: port || this.options.server.port,
+      port: isNaN(parseInt(port)) ? this.options.server.port : port,
       host: host || this.options.server.host,
       socket: socket || this.options.server.socket,
       https: this.options.server.https,

test/unit/server.listen.test.js

@@ -41,4 +41,33 @@ describe('server listen', () => {
 
     await nuxt.close()
   })
+
+  test('should skip the use of default port when listening on port 0', async () => {
+    // Stub process.env.PORT
+    const stubDetails = {
+      originalValue: process.env.PORT,
+      hasProperty: 'PORT' in process.env
+    }
+    const DEFAULT_PORT = '2999'
+    process.env.PORT = DEFAULT_PORT
+
+    // Setup test
+    const nuxt = new Nuxt({ ...config, dev: true })
+    const listen = () => nuxt.server.listen(0, 'localhost') // Use port 0 to let allow host to randomly assign a free PORT
+    const toString = (x = '') => `${x}`
+
+    // Nuxt server should not be listening on the DEFAULT_PORT
+    await listen()
+    expect(toString(nuxt.server.listeners[0].port)).not.toBe(DEFAULT_PORT)
+
+    // Reset stub for process.env.PORT
+    if (stubDetails.hasProperty) {
+      process.env.PORT = stubDetails.originalValue
+    } else {
+      delete process.env.PORT
+    }
+
+    // Finalize test
+    await nuxt.close()
+  })
 })