When may the JavaScript exception 'TypeError: "x" is not a function' occur?

The JavaScript exception "TypeError: "x" is not a function" occurs when the type of the value returned by function V does not match its declared type. For example, if a function is declared to return an integer but returns a string instead, this exception will occur. This can happen when incorrect data types are used or when the function is called with the wrong number of arguments. Reference:https://www.adobe.com/content/dam/acom/en/devnet/analytics/analytics-developer-professional-study-guide.pdf