出现google未定义，求解，代码如下

<!DOCTYPE html>
<html dir="LTR">
<head>
    <meta name="viewport" content="initial-scale=1.0, user-scalable=no" />
    <meta http-equiv="content-type" content="text/html; charset=UTF-8" />
    <style type="text/css">
        html
        {
            height: 100%;
        }
        body
        {
            height: 100%;
            margin: 0px;
            padding: 0px;
        }
        #map_canvas
        {
            height: 600px;
            width: 600px;
        }
    </style>
    <title>Google Maps</title>
    <script type="text/javascript" src="http://maps.google.com/maps/api/js?sensor=false"></script>
    <script type="text/javascript">
        var map;
        function initialize() {
            var myLatlng = new google.maps.LatLng(100.300, 120.600);
            var myOptions = {
                zoom: 8,
                center: myLatlng,
                mapTypeId: google.maps.MapTypeId.ROADMAP
            }
            map = new google.maps.Map(document.getElementById("map_canvas"), myOptions);
        }//        function loadScript() {
//            var script = document.createElement("script");
//            script.type = "text/javascript";
//            script.src = "http://maps.google.com/maps/api/js?sensor=false&callback=initialize";
//            document.body.appendChild(script);
//        }        //window.onload = loadScript;
    </script>
</head>
<body onload="initialize()">
    <div id="map_canvas">
    </div>
</body>
</html>
但是，如果把loadScript（）解注释掉，同时，解注释掉window.onload那句，注释掉<script type="text/javascript" src="http://maps.google.com/maps/api/js?sensor=false"></script>这句，就可以在浏览器和桌面应用程序中加载地图，求解。