A variable named test is different from a variable named Test.
typeof can't be the name of a function, because it's a keyword; however, typeOf is a valid function name.
An identifier is the name of a variable, function, property, or function argument.
Identifiers may be one or more characters in the following format:
Letters in an identifier may include extended ASCII or Unicode letter characters, though this is not recommended.
firstSecond myCar doSomethingImportant
Keywords, reserved words, true, false, and null cannot be used as identifiers.