配列でもオブジェクトでも繰り返ししてくれるforEach

var forEach = function(list, callback) { 
  if ( list instanceof Array ) { 
    var len = list.length; 
    for (var i=0; i<len; i++) { 
      callback.call(list[i], i, list[i]); 
    }   
  } else {
    for (var key in list ) if ( list.hasOwnProperty(key) ) { 
      callback.call(list[key], key, list[key]); 
    }   
  }   
}; 

// 配列  
var arr = ["hoo","bar","baz"]; 
forEach(arr, function(i, val) { 
  console.log(i + ":" + val); 
}); 
// オブジェクト  
var obj = { jp: "japan", us: "united states" };  
forEach(obj, function(key, val) { 
  console.log(key + ":" + val + "=" + this); 
});