typeof {}; // "object" 
typeof []; // "object" 
typeof new Date(); // "object" 

[] instanceof Array; // true 
[] instanceof Object; // true 
[] instanceof RegExp; // false 
new Date instanceof Date; // true 

function isArray(arr){ 
  return arr instanceof Array; 
} 

function isArray(arr){ 
  return typeof arr == "object" && arr.constructor == Array; 
} 

<script>
window.onload=function(){
var iframe_arr=new window.frames[0].Array;
alert(iframe_arr instanceof Array); // false
alert(iframe_arr.constructor == Array); // false
}
</script>

function isArray(arr){
  return Object.prototype.toString.call(arr) === "[object Array]";
}

/**
 * Returns internal [[Class]] property of an object
 *
 * Ecma-262, 15.2.4.2
 * Object.prototype.toString( )
 *
 * When the toString method is called, the following steps are taken: 
 * 1. Get the [[Class]] property of this object. 
 * 2. Compute a string value by concatenating the three strings "[object ", Result (1), and "]". 
 * 3. Return Result (2).
 *
 * __getClass(5); // => "Number"
 * __getClass({}); // => "Object"
 * __getClass(/foo/); // => "RegExp"
 * __getClass(''); // => "String"
 * __getClass(true); // => "Boolean"
 * __getClass([]); // => "Array"
 * __getClass(undefined); // => "Window"
 * __getClass(Element); // => "Constructor"
 *
 */
function __getClass(object){
  return Object.prototype.toString.call(object).match(/^\[object\s(.*)\]$/)[1];
};

var is ={
  types : ["Array", "Boolean", "Date", "Number", "Object", "RegExp", "String", "Window", "HTMLDocument"]
};
for(var i = 0, c; c = is.types[i ++ ]; ){
  is[c] = (function(type){
    return function(obj){
      return Object.prototype.toString.call(obj) == "[object " + type + "]";
    }
  )(c);
}
alert(is.Array([])); // true
alert(is.Date(new Date)); // true
alert(is.RegExp(/reg/ig)); // true