I been writing JavaScript recently and had to conjure up the older syntax I'd usually write in ActionScript 2
This post is possibly "beating a dead horse" but I'll post it anyway.

JavaScript or ActionScript 2

function isArray(o)
{            
    return o instanceof Array;
 
    //Alternative way
    //return Object(o.constructor) == [].constructor;
}
 
var A=new Array();
var B=new Object();
var C="dog";
var D=function(){};
 
alert(isArray(A)+","+isArray(B)+","+isArray(C)+","+isArray(D));
 
//Outputs: true,false,false,false

ActionScript 3

function isArray(o:Object=null):Boolean
{
    return o is Array;
}
 
var A:Array=new Array();
var B:Object=new Object();
var C:String="dog";
var D:Function=function(){};
 
trace(isArray(A)+","+isArray(B)+","+isArray(C)+","+isArray(D));
 
//Outputs: true,false,false,false