Javascript identifiers are used for naming functions and variables. These must be defined in accordance with the following rules:

1. must not be a Javascript reserved word (list)
2. may contain letters, digits (0 to 9), dollar signs ($) and underscores (_)
3. must start with a letter, a dollar sign ($) or an underscore (_)
4. must be case-sensitively unique to its context (you cannot have two variables with the same name)