lang-as3

name	lang-as3
description	AS3 / AIR 51 language gotchas — optional chaining bugs, immutability limits, event dispatch semantics

AS3 / AIR 51 — Verified Gotchas

Optional chaining ?.

• Property access: obj?.prop — works, returns undefined if null
• Method calls: obj?.method() — BROKEN. Calls null() instead of short-circuiting. Use if (obj != null) obj.method()
• When reverting a broken ?. call to an explicit null-check, leave a marker comment at the site (e.g. // ?. AIR bugfix — don't restore optional chaining) so later edits don't reintroduce the bug

Immutability

• final only works on methods and classes, NOT on fields — final var is a compile error
• Use const for never-reassigned fields instead: private const _items:Array = []; — assign at the declaration, or once in the constructor
• const is shallow — the binding is fixed, the object's contents stay mutable

removeEventListener does not take effect during an in-flight dispatch

Adobe docs (EventDispatcher): "If an event listener is removed from a node while an event is being processed on the node, it is still triggered by the current actions." The listener list is snapshotted when the dispatch starts.

• Listener added during dispatch → not called in the current phase.
• Listener removed during dispatch → still called in the current dispatch.

Consequence: a handler can run after its own cleanup already nulled the fields it reads → TypeError #1009. Classic shape — many objects listen to stage ENTER_FRAME; handler A synchronously disposes object B; B's handler is still in the snapshot and fires with _field == null.

Removing a listener is therefore not a sufficient guard. Any handler on a shared dispatcher must also null-check the state it touches, or check a disposed flag first.

as operator with primitive types

• context.call(...) as int works only if the native extension returns FRENewObjectFromInt32 (value arrives as a boxed int). If the C++ side returns FRENewObjectFromDouble, as int silently returns null, which coerces to 0.
• Prefer explicit cast: int(context.call(...)) — works regardless of whether the native side returns int, uint, Number, or double.
• as is designed for reference types. With primitives (int, uint, Number, Boolean), behavior depends on the exact runtime type of the boxed value — fragile and non-obvious.