使用Array的原型使对象具有length，和数组的内容

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