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:
- The first character must be a letter, an underscore (_), or a dollar sign ($).
- All other characters may be letters, underscores, dollar signs, or numbers.
- Letters in an identifier may include extended ASCII or Unicode letter characters, though this is not recommended.
Keywords, reserved words, true, false, and null cannot be used as identifiers.