<script>

    function baseClass()

    {

        this.showMsg = function()

        {

            alert("baseClass::showMsg");

        }

    }

    function extendClass()

    {

        this.showMsg =function ()

        {

            alert("extendClass::showMsg");

        }

    }

    extendClass.prototype = new baseClass();

    var instance = new extendClass();

    instance.showMsg();

    //显示extendClass::showMsg

</script>