.AS File Format – Statements

ActionScript 2.0

Method bodies, frame scripts and handlers are lists of statements. ActionScript 2.0 has every statement of ActionScript 1.0 in Flash MX and adds exception handling. The declarations that make up a class file are described with Classes, Interfaces and Packages.

Grammar

expr is any expression, stmt one statement or a block in braces, type a class or interface name, and square brackets mark optional parts.

statement  = block | var-stmt | expr ";" | if-stmt | loop | for-in | switch-stmt
           | "break" ";" | "continue" ";" | "return" [expr] ";"
           | function-decl | with-stmt | try-stmt | "throw" expr ";" | ";"
block      = "{" { statement } "}"
var-stmt   = "var" name [":" type] ["=" expr] { "," name [":" type] ["=" expr] } ";"
if-stmt    = "if" "(" expr ")" stmt [ "else" stmt ]
loop       = "while" "(" expr ")" stmt
           | "do" stmt "while" "(" expr ")" ";"
           | "for" "(" [init] ";" [expr] ";" [expr] ")" stmt
for-in     = "for" "(" ["var"] name [":" type] "in" expr ")" stmt
switch-stmt = "switch" "(" expr ")" "{" { "case" expr ":" { statement } }
              [ "default" ":" { statement } ] "}"
function-decl = "function" name "(" [param { "," param }] ")" [":" type] block
param      = name [":" type]
with-stmt  = "with" "(" expr ")" stmt
try-stmt   = "try" block { "catch" "(" name [":" type] ")" block } [ "finally" block ]
The statements of ActionScript 2.0. A try needs at least one catch or a finally.

Every statement

StatementDoesPlayer
varDeclares variables, optionally typed and given starting values.5
if … elseRuns one statement or the other by a condition.4
while, do … whileRepeat while a condition is true, testing before or after each pass.4
forThe counting loop.5
for … inRuns once for each enumerable property name of an object. The methods of ActionScript 2.0 classes are hidden from it.5
break, continueLeave the innermost loop or switch, or start the loop’s next pass. There are no labels.4
returnLeaves a function with a value, which must suit its declared return type.5
switch, case, defaultJumps to the matching case, or to default, and runs on until break.4, 6
withPuts an object at the front of the scope chain for its body.5
functionDeclares a named function; see Functions.5
try, catch, finallyRuns a block, catches values thrown in it, and runs the finally block whatever happens.7
throwThrows a value, usually an Error or a subclass of it.7
tellTarget, ifFrameLoadedDeprecated Flash 4 statements, still accepted in FLA scripts.3, 4

Exceptions

Any value can be thrown, but the convention is an Error object, which has name and message properties, or an instance of a subclass of Error. A try block can have several catch blocks with typed parameters; the first whose type fits the thrown value runs, and an untyped one catches everything. An exception that nothing catches stops the rest of the script it was thrown in.

class com.example.data.ParseError extends Error {
    public var line:Number;
    public function ParseError(message:String, line:Number) {
        super(message);
        this.name = "ParseError";
        this.line = line;
    }
}

// elsewhere
try {
    settings = parser.read(text);
} catch (e:ParseError) {
    trace("Bad settings on line " + e.line);
} catch (e) {
    trace("Unexpected: " + e);
} finally {
    loading_mc._visible = false;
}
An error class of its own, and a try with two catch blocks.

Flash Player does not throw exceptions for mistakes in ActionScript 2.0 code, such as calling a method that does not exist or reading a property of undefined; those still fail quietly. Only throw statements, in user code or in classes such as the version 2 components, throw. Exceptions need a SWF file for Flash Player 7 or later.

Handlers and directives

In FLA scripts only, as in ActionScript 1.0:

ConstructWhere
on (event) { … }The script of a button or movie clip instance; see Events
onClipEvent (event) { … }The script of a movie clip instance
#include "file.as"Any FLA script or include file, but not a class file
#initclip … #endinitclipThe first frame of a symbol; with ActionScript 2.0 rarely written by hand, since the compiler makes these blocks for classes