Flash Player Oddities

This page is a list of weird behaviors in the official Flash Player that we don't quite understand and require investigation. This also includes behavior that varies between different version of the Flash Player. Spooky! 👻

AVM1

DefineFunction2 skips preloading _parent register

If a DefineFunction2 function on the root timeline tries to preload _parent in a register, the player ends up skipping this preload because _parent is undefined. _global will mistakenly get preloaded in this register instead if the function also uses it. See the define_function2_preload_order test for an example.

NaN == NaN is true

NaN == NaN returns true in AVM1 depending on the version of the Flash Player. Flash Player 6 and below(?) properly returns false. Higher versions return true.

NaN.toString(base) returns garbage for non-decimal bases

NaN.toString(16) returns -(0000000 for example. There are different garbage values for every non-decimal base. The int-to-string algorithm ends up chewing on the bit pattern for -NaN (0xFFF80000_00000000), getting negative values for the digit and indexing the wrong way from ASCII '0'.

Some simple code that should match the result of Flash Player for NaN: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=95ff82ad6762025e08556ace0e061ae0

This depends on the version of the Flash Player, regardless of SWF version. It seems to occur Flash Player 7? and higher. Flash Player 6 and below prints 0 for all non-decimal bases. Currently Ruffle returns 0 in this case, but we may want to make this a setting in the future.

o is null

trace(o) will print null instead of the expected undefined.