MAY 2020 - You don't have to add any javascript files or config files anywhere. Let me explain.
I was facing this exact same issue and wasted 6 hours trying everything including the most popular answer to this question. While the accepted answer is a nice workaround (but requires more work than just adding the index.js file), there's something a simpler than that.
You see, when you just deploy an Azure Web App (or App Service as it is also called), two things happen:
The web app by default points to opt/startup/hostingstart.html
It also puts a hostingstart.html in home/site/wwwroot
When you deploy your code, it replaces hostingstart.html in home/site/wwwroot but the app is still pointing to opt/startup/hostingstart.html. If you want to verify this, try deleting opt/startup/hostingstart.html file and your web app will throw a "CANNOT GET/" error.
So how to change the default pointer? It's simpler than it looks:
Go to Configuration tab on your web app and add the following code to startup script:
pm2 serve /home/site/wwwroot --no-daemon
If this web app is a client-side single-page-app and you're having issues with routing, then add --spa to the above command as follows:
pm2 serve /home/site/wwwroot --no-daemon --spa
This will tell the web app to serve wwwroot folder. And that's it.
Image for reference:
Screenshot explaination
PS: If you only set the startup script without deploying your code, it will still show the hostingstart.html because by default that file lies in the wwwroot folder.
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