| 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.
