In my case, running on Windows 10, my problem turns out to be the folder name where the files were located. I used a root folder of “c:!dev” (so as to always see the folder at the top of a Windows Explorer). The “!” was the problem, when changing the folder to “c:\dev”, “ionic serve” was able to build the main.js file and properly display the page. Not sure why, but hope this helps anyone who might not think of the folder name being part of the problem.