JavaScript: The Definitive Guide, Fourth Edition

Labels

The case and default: labels used in conjunction with the switch statement are a special case of a more general label statement. In JavaScript 1.2, any statement may be labeled by preceding it with an identifier name and a colon:

               identifier: statement

The identifier can be any legal JavaScript identifier that is not a reserved word. Label names are distinct from variable and function names, so you do not need to worry about name collisions if you give a label the same name as a variable or function. Here is an example of a labeled while statement:

parser:
  while(token != null) {
      // Code omitted here
}

By labeling a statement, you give it a name that you can use to refer to it elsewhere in your program. You can label any statement, although the only statements that are commonly labeled are loops: while, do/while, for, and for/in. By giving a loop a name, you can use break and continue to exit the loop or to exit a single iteration of the loop.