When working on web apps we have to keep in mind that the client might not have access to the internet all the time. Chrome has a great feature for this:
When working on a web app that make requests to a API located at localhost but also make requests to external API:s, it’s harder.
I solved this issue by creating a new Location in OS X Network part of System Preferences. The new Location has WiFi and Ethernet Configure IPv4 set to Off, which means that it’s offline but you can still reach your localhost server.