I upgraded my Ionic 4 application by these two commands:
# Upgrading to Ionic 5
npm install @ionic/angular@latest @ionic/angular-toolkit@latest --save
# Upgrading to Angular 9
ng update @angular/core @angular/cli
And when I did ionic serve, I started getting this error:
> ng run app:serve --host=localhost --port=8100
[ng] chunk {} 0.js, 0.js.map () 22.1 kB [rendered]
[ng] chunk {common} common.js, common.js.map (common) 25.7 kB [rendered]
[ng]
[ng] ERROR in ./src/zone-flags.ts
[ng] Module build failed (from ./node_modules/@ngtools/webpack/src/index.js):
[ng] Error: /Users/shashank/Projects/my-app/mobile-app/src/zone-flags.ts is missing from the TypeScript compilation. Please make sure it is in your tsconfig via the 'files' or 'include' property.
[ng] at AngularCompilerPlugin.getCompiledFile (/Users/shashank/Projects/my-app/mobile-app/node_modules/@ngtools/webpack/src/angular_compiler_plugin.js:909:23)
[ng] at plugin.done.then (/Users/shashank/Projects/my-app/mobile-app/node_modules/@ngtools/webpack/src/loader.js:41:31)
[ng] at process._tickCallback (internal/process/next_tick.js:68:7)
The fix is to change the following line in mobile-app/src/polyfills.ts:
From:
import './zone-flags.ts';
To:
import './zone-flags';