If the script is called app.js and stored in the js folder of you app, it get loaded automatic.
If it’s called different, that is NOT the case and you have to load it manually:
Have you created an app.js from your app.ts? Not having done so, could also lead to the error you mentioned.
Probably more helpful would be, if you post some code fragments, or a link to your code on GitHub, if it’s public.