Ruby on Rails with AngularJS: "Argument [...] is not a function, got undefined"

I have just spent several hours trying to figure out what is wrong with my newly created application.

Error: Argument [...] is not a function, got undefined

It turns out that problem... can be anywhere.

What's wrong?

It usually means we have forgotten about some braces, colons, semicolons, commas, dots etc. In short, there is a syntax error.

But I have been reading my code very carefully and there was no mistake.

What worked for me?

I named my main javascript file main.js. Don't do it.

Name it app.js instead and everything should be fine.