rly simple self-defining test function snippet

is_a.js

/* Just define a simple is_a fn and populate it with functions
 * that correspond to the primary JS objects; interface is
 *
 * is_a.array([]) >> true
 */
;(function (window, undefined){
  var upFirst = /^(\w)/, 
      toS = Object.prototype.toString,
      builtIns = ['Array', 'Object', 'Number', 'String', 'Window', 'RegExp', 'Function'];
  
  function up(str) {
    return str.replace(upFirst, function($0, $1){ return $1.toUpperCase();});
  }
  
  function makeTest(_type) {
    _type = up(_type);
    return function (o) {
      toS.call(o) === "[object " + _type + "]"; 
    }
  }
  
  function is_a(_type, obj) {
    return (is_a[_type] = (is_a[_type] !== undefined ? is_a[_type] : makeTest(_type)))(obj);
  }
  
  // set up all the built ins, attach is_a to the window
  var i = 0, len = builtIns.length;
  for(; i < len; i++) {
    is_a(builtIns[i]);
  }
  
  window.is_a = is_a;
}(window)); 

// usage
is_a('function', fn); 
is_a.Function(fn);
is_a.Number(num);