I ran into an issue where I really needed to tag the built-in Javascript types with additional information to make Fan work better. The big one is annotating a Number to be either a Int or Float, which you need to do to make all the equality checks work consistently (since 5 != 5f).
This is was my first attempt, which I assumed would work:
>>> var x = 15;
>>> x.$fanType = sys_Type.find("sys::Int");
>>> x.$fanType
undefined
But no dice. After a bit of googling, I found this page, which says in order to add properties to the built-in types, you must create objects using the constructor syntax:
>>> var x = new Number(15);
>>> x.$fanType = sys_Type.find("sys::Int");
>>> x.$fanType
"sys::Int"
I assume this a performance optimization, where you can use a primitive int value in the first case, and have to allocate an object in the latter. Though since Javascript is a pure-OO language, I would be interested in seeing how VMs actually implement this behavoir.
Edit 8 Jun 09: Just realized using the object syntax breaks equality checks for numbers:
>>> var x = new Number(4); >>> var y = new Number(4); >>> x == y false
Which sort of sucks, but you can work around it using valueOf, but still annoying.
>>> x.valueOf() == y.valueOf() true
I stumbled across an interesting “edge” case in Javascript when porting gfx to Javascript.
Turns out Javascript converts numbers to 32-bit signed numbers before evaluating bitwise operators. This is problematic because it prevents you from using the msb for 32-bit bitmaks:
Java: 0xaabbcc | 0xff000000 = 0xffaabbcc (4289379276) Javascript: 0xaabbcc | 0xff000000 = 0xffabbcc (-554434)
The workaround is pretty straightforward, you simply have to add 2^32+1 to convert back to an “unsigned” value:
var x = 0xaabbcc | 0xff000000; if (x < 0) x += 0xffffffff+1;
But unfortunately, that means I have to wrap all bitwise operations in a method call, and perform that check, which I expect will not be anywhere near as fast as the straight operator call.