Nowadays, it's more important than ever to ensure that websites and web applications look and feel great both on a desktop and on a mobile device. While many tools have been created over the years to help aid in mobile-responsiveness, such as Responsively and Chrome's built-in Inspect tool, sometimes it would make things a lot easier and smoother to just see your progress, live, from your phone, without having to deploy a work-in-progress.
Fortunately, there is an easy way to view the same localhost you see in your browser, on your smartphone.
Prerequisite: Your phone and your machine must be connected to the same wifi signal for this to work.
Once you've located that address (it should look like 188.8.131.52, but with different numbers), enter it in your phone's mobile browser (Chrome, Safari, whatever) address bar, followed by the port you previously opened on your machine (localhost:3000 or 8000 or whatever it may be). So the final result will look something like this:
As long as your localhost port is running and you entered the numbers accurately, your phone should now display the mobile view of your project. It should reload in sync with your desktop as you make changes and save.
Congratulations! You now have a live view of what your project will look like on your particular mobile device, without having to use additional tools, install special software, or deploy a half-made app. While this method doesn't come with any special development tools, it does help ensure that some of the inconsistencies caused by viewing on "mobile" through other means can be eliminated.