if relevant for now.
in Russian:
https://learn.javascript.ru/native-prototypes
in English:
https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Objects/Object_prototypes

Please mark your post as having spoiler content next time. Read the description:

Define String.prototype.toAlternatingCase ...

See the initial code:

String.prototype.toAlternatingCase = function () {
  // Define your method here :)
}