When dealing with functions, we often run into illogical cases. We can address these by "throwing an Error" with an error message that describes the problem:
throw new Error("Error message");
For example, if I want to deal with the illogical case of trying to access the fifth element in a List that's only four elements long, I might throw this Error:
throw new Error("Cannot access an element beyond the length of the List");
Throwing an Error causes the program to stop and display the error message you provide.