You may be missing a development configuration for your application.

In your angular.json navigate to architect -> build -> configurations and add these lines, below the definition of the production build:

"configurations": {
  "production": {
    // already there!
  },
  "development": { // this is new!
    "buildOptimizer": false,
    "optimization": false,
    "vendorChunk": true,
    "extractLicenses": false,
    "sourceMap": true,
    "namedChunks": true
  }
},
"defaultConfiguration": "production"

And to run the build in development mode just type the next command: ng build --configuration development.

The specs have been updated since that article (I think they now go down to IE9), but Opera Mini has really shoddy javascript support so using any client-side framework for it is a bad idea. They may work since they do have some support, but don't count on it.

In general, if 80% of your users are on opera mini, don't use a SPA. In fact, you may want to avoid javascript completely, using something like Django would be a much safer option and it would probably result in a smaller page for the user to download (and saving on data is one of the prime reasons people use Opera mini).

You can use special commands(enter command into address panel):

server:console - Show console and javascript error for current page

server:source - Display the current page's source code

server:reset - clears all cookies

config: - Open the the Power User Settings menu

opera:cache - Displays the size and URL of cached pages and images

about: - Displays information about the Opera Mini server

debug: - Displays a page listing many Opera mini server and client properties. The "Host:" field tells you which Opera Mini server you are currently connected to.