YUI之数组操作

YUI的构建数组，将类数组转换成真正的数组，从而可以使用数组的所有方法

   //真正的数组返回1，类数组返回2，其余的返回0
    YArray.test = function (obj) {
    var result = 0;

    if (Lang.isArray(obj)) {
        result = 1;
    } else if (Lang.isObject(obj)) {
        try {
            // indexed, but no tagName (element) or scrollTo/document (window. From DOM.isWindow test which we can't use here),
            // or functions without apply/call (Safari
            // HTMLElementCollection bug).
            if ('length' in obj && !obj.tagName && !(obj.scrollTo && obj.document) && !obj.apply) {
                result = 2;
            }
        } catch (ex) {}
    }

    return result;
   };

  Native = Array.prototype, hasOwn = Object.prototype.hasOwnProperty; //thing  要构建数组的元素  startIndex  从thing中的第几个开始构建数组  force强制转换数组，thing将会当成一个类数组  
   //Note: elements that are also collections, such as `<form>` and `<select>`
  //elements, are not automatically converted to arrays. To force a conversion,
  //pass `true` as the value of the _force_ parameter.

  function YArray(thing, startIndex, force) {
    var len, result;

    /*jshint expr: true*/
    startIndex || (startIndex = 0);

    if (force || YArray.test(thing)) {
        //IE8及以下的会报错，thing在这里是NodeList对象，不是javascript对象，在现代浏览器已除去这个限制
        try {
            return Native.slice.call(thing, startIndex);
        } catch (ex) {
            result = [];

            for (len = thing.length; startIndex < len; ++startIndex) {
                result.push(thing[startIndex]);
            }

            return result;
        }
    }

    return [thing];
  } 
  Y.Array = YArray; 
  var aA = document.getElementsByTagName(‘a’) 
  var html = document.getElementsByTagName(‘html’)[0]
  YArray(aA)  ==>  传入一个类数组，产生一个真正的数组  
  YArray(html)  ==>  传入一个对象，产一个数组 
  YArray(1,2,3,4,5) ==> 传入一系列的值，产生一个空的数组 
  YArray(1,0,0,0)  ==>  如果第三个参数为假，则产生[第一个参数]的数组

a.  ‘length’  in obj 与length in obj的区别

        在数组中这两个没有区别

        在对象中这两个有区别，'length'表示obj中有'length'这一个键，length表示obj的属性,因此 'length’ in window ==> true     而 length in window  ==> false