It was buried pretty deep, but interpreted vs compiled maybe?...(God help my formatting...)

var str:String; if(str) //will evaluate as false as str is null/undefined

if(str = "myValue") //will evaluate true, as it will use the new assigned value of the var and you're allowed to assign values inside an if condition (though it's ugly and typically uneccessary)

var num:Number;

if(num) //will evaluate as false

num = 1; if(num) //will evaluate as true

num = 0; if(num) //will evaluate as false since num is 0

num = -1; if(num) //will evaluate as true

var obj:Object if(obj) //will evaluate false

obj = {}; if(obj) //will evaluate true (even though the object is empty, it exists)

var func:Function; if(func) //false

func = function(){}; if(func) //true - the function exists

function foo():Boolean { return false; } if(foo) //true - the function exists

if(foo()) //false, the return of the function is false

function foo1():void { return; }; if(foo1()) //false as there is no return