Symptoms Probable Cause Action
Unable to load the localhost page in browser Your URL does not match your port number. Double check what port you have set in server.js and if your browser URL is localhost:PORT with PORT being the number you have set.
You forgot to tell the Express app to listen to a port number. Make sure you have app.listen(port, () => {}) in server.js to configure which port you want Express to watch.
You have another app running on the same port. Close any additional servers that might be running. Only one app per port is possible.
ESLint warns you about the use of the console You have not configured ESLint to accept console.log. Make sure you have rules: {'no-console': 'off' }, set in .eslintrc.js as directed in Eslint Customization
TypeError: app.listen is not a function or similar You forgot to export one of your modules (ie. app or router). Make sure you have module.exports = router; at the end of your router files and module.exports = app; where you created your new Express app.
Error: Cannot find module <NAME> You're requiring the module from the wrong path. Double check the path in the require(PATH) is pointing to the correct location.
localhost is loading indefinitely You forgot to include (or you are not hitting) a response method. Make sure the handler function includes a response method to end the HTTP request-response cycle.