使用Array的原型使对象具有length，和数组的内容

var elems = {
length: 0,
add: function (elem) {
Array.prototype.push.call(this, elem);
},
gather: function (id) {
this.add(document.getElementById(id));
}
};
elems.gather("first");
console.log(elems.length == 1 && elems[0].id, "Verify that we have an element in our stash");
elems.gather("second");
console.assert(elems.length == 2 && elems[1].id, "Verify the other insertion");