.AS File Format – Lexical structure

White space and semicolons

Spaces, tabs and line breaks separate words and are otherwise ignored, except inside strings. A semicolon ends a statement. It may be left out at the end of a line, where the compiler takes the line break as the end, but the guides of the time advised always writing it. Some rules follow from this:

• return followed by a line break returns undefined; the value must start on the same line.
• A block in braces, as after if, for or function name(), needs no semicolon after its closing brace.
• An assignment of a function literal, f = function () { … };, is a statement and ends with a semicolon.
• Compiler directives such as #include stand on a line of their own and take no semicolon.

Comments

Identifiers

Names of variables, functions, properties and objects are made of letters A to Z and a to z, digits, _ and $, and may not start with a digit. A name may not be a reserved word. There is no set length limit.

Flash’s own properties start with an underscore: _x, _alpha, _root, _parent. Names that need other characters, or are built while the movie runs, can still be used as property names with square brackets, this["item" + i], or with eval and set. Instance names of movie clips, frame labels and layer names follow no rules of their own, but only those that are valid identifiers can be written in dot syntax.

Case sensitivity

ActionScript 1.0 differs from ECMA-262 here, to keep Flash 4 movies working:

• Keywords are case-sensitive. onClipEvent, function and if must be written exactly so; OnClipEvent or If is an error.
• Everything else is not. Variable, property and function names, built-in class names, instance names, frame labels and symbol linkage names are matched without regard to case when the movie runs in a SWF file for Flash Player 6 or earlier, which is everything Flash 5 and Flash MX make. myScore, MyScore and myscore are one variable, and new array() makes an Array.
• A consequence is that a variable can hide a class: after date = new Date(), the name Date on that timeline means the variable, not the constructor. Names such as myDate avoid it.

The ActionScript 1.0 compiler does not check case either. The same code published as a SWF for Flash Player 7, which Flash MX 2004 can do, becomes case-sensitive when it runs, so old code with mixed spellings of one name breaks there.

Reserved words

These words are keywords in Flash 5 and cannot be used as names. The ones marked Flash 4 are the old operators and actions, kept but deprecated.

Flash MX adds switch, case, default, instanceof and super. The values true, false, null, undefined, NaN, Infinity and newline are built-in constants.

These words were set aside for the future and should not be used as names: abstract, boolean, byte, catch, char, class, const, debugger, double, enum, export, extends, final, finally, float, goto, implements, import, int, interface, long, native, package, private, protected, public, short, static, synchronized, throws, transient, try and volatile. Several became keywords of ActionScript 2.0.

Literals