declare variables in for-statement

Compiler catalogues (stores in its internal "database") variables in scopes before runtime. Thus, the d declared in the loop is actually catalogued and declared BEFORE loop execution in the scope of the class that handles timeline which is internal by default. Compiler tolerates duplicate variable declarations in other than class scopes and just gives warning. In other than class scopes compiler declares variable just once for you and overrides duplicates.

In other words - compiler knows already about var d before first trace shows up. Since the variable is int - its default value is 0 - what you see in the trace. If d is Number - first trace will be NaN, if any other datatype - it will be null.

The fact that class scopes do not allow for duplicate variable declarations probably stems from the same root as the fact that there is no overloading in AS3 unlike in other languages.

Another illustration:

var d:Number = 100;
testMe();
function testMe():void
{
          for (var i:int = 0; i < 2; i++)
          {
                    trace(d);
                    var d:String = "s";
          }
}

In this case first trace is null although one might expect 100. This is because in the function d is property of function testMe() - not class scope (or namespace).