March 1999 Draft JavaScript 2.0 Tokens

Monday, March 22, 1999

Punctuators

The following JavaScript 1.x punctuation tokens are recognized in JavaScript 2.0:

!   !=   !==   %   %=   &   &&   &=   (   )   *   *=   +   ++   +=   ,   -   --   -=   .   ..   ...   /   /=   :   ::   ;   <   <<   <<=   <=   =   ==   ===   >   >=   >>   >>=   >>>   >>>=   ?   [   ]   ^   ^=   {   |   |=   ||   }   ~

The following punctuation tokens are new in JavaScript 2.0:

#   &&=   ->   ..   ...   @   ^^   ^^=   ||=

Keywords

The following reserved words are used in JavaScript 2.0:

break   case   catch   class   const   continue   default   delete   do   else   eval   extends   false   field   final   finally   for   function   if   import   in   instanceof   new   null   package   private   protected   public   return   super   switch   this   throw   true   try   typeof   var   while   with

Out of these, the only words that were not reserved in JavaScript 1.x are eval and field.

The following reserved words are reserved for future expansion:

abstract   debugger   enum   export   goto   implements   native   static   synchronized   throws   transient   volatile

The following words have special meaning in some contexts in JavaScript 2.0 but are not reserved and may be used as identifiers:

constructor   getter   method   override   setter   traditional   version

The following words name predefined types but are not reserved and may be used as identifiers (although this is not recommended):

Any   Array   array   boolean   character   Function   integer   Null   number   Object   object   string   Type   type   void

Semicolon Insertion

General semicolon insertion at any line break cannot be supported in JavaScript 2.0 because it would introduce too many unexpected program behaviors and break future compatibility (a program with lines LINE1 and LINE2 separated by a line break might be interpreted as LINE1;LINE2 today, while a future syntax extension might change its meaning to LINE1 LINE2). For example, the program

var x
i = 3

would be interpreted as

var x i = 3

and treat x as a type expression for i's declaration.

However, the JavaScript 2.0 grammar makes semicolons optional in the following situations:

• Before any }
• Before the else of an if-else statement
• Before the while of a do-while statement (but not before the while of a while statement)
• Before the end of the program

Semicolons are optional in these situations even if they would construct empty statements.

Line breaks are no longer significant in source code and are treated like any other white space. Special JavaScript 1.x grammar productions that forbid line breaks after a return or prefix ++ or -- now allow line breaks in those places.

Regular Expression Literals

To support error recovery, JavaScript 2.0's lexical grammar must be made independent of its syntactic grammar. To make the lexical grammar independent of the syntactic grammar, JavaScript 2.0 determines whether a / starts a regular expression or is a division (or /=) operator solely based on the previous token:

Regardless of the previous token, // is interpreted as the beginning of a comment.

The only controversial choices are ) and }. A / after either a ) or } token can be either a division symbol (if the ) or } closes a subexpression or an object literal) or a regular expression token (if the ) or } closes a preceding statement or an if, while, or for expression). Having / be interpreted as a RegularExpression in expressions such as (x+y)/2 would be problematic, so it is interpreted as a division operator after ) or }. If one wants to place a regular expression literal at the very beginning of an expression statement, it's best to put the regular expression in parentheses. Fortunately, this is not common since one usually assigns the result of the regular expression operation to a variable.

A RegularExpression can also be specified unambiguously using « and » as its opening and closing delimiters instead of / and /. For example, «3*» is a regular expression that matches zero or more 3's. Such a regular expression can be empty: «» is a regular expression that matches only the empty string, while // starts a comment.

Making JavaScript 2.0's lexical grammar independent of its syntactic grammar significantly simplifies the language, removes many ambiguities, and allow tools to easily process a JavaScript program and escape all instances of, say, </ to properly embed a JavaScript 2.0 program in an HTML page. In JavaScript 1.x such a tool is not practical because it requries a full parser of the langauge to distinguish an unquoted </ from one inside a string or one in a regular expression. The full parser changes for each version of JavaScript. To illustrate the difficulties, compare such JavaScript 1.4 gems as:

for (var x = a in foo && "</x>" || mot ? z:/x:3;x<5;y</g/i) {xyz(x++);}
for (var x = a in foo && "</x>" || mot ? z/x:3;x<5;y</g/i) {xyz(x++);}

Waldemar Horwat Last modified Monday, March 22, 1999