移动到外部.js文件时JavaScript无法正常工作

Question

移动到外部.js文件时JavaScript无法正常工作

我在我的html页面中编写了一个java脚本.现在我将脚本移动到外部js文件.这是脚本(jful.js)

<script type="text/javascript">
$(function() {

    // grab the initial top offset of the navigation 
    var sticky_navigation_offset_top = $('#catnav').offset().top;

    // our function that decides weather the navigation bar should have "fixed" css position or not.
    var sticky_navigation = function(){
        var scroll_top = $(window).scrollTop(); // our current vertical position from the top

        // if we've scrolled more than the navigation, change its position to fixed to stick to top,
        // otherwise change it back to relative
        if (scroll_top > sticky_navigation_offset_top) { 
            $('#catnav').css({ 'position': 'fixed', 'top':0, 'left':0 });
        } else {
            $('#catnav').css({ 'position': 'relative' }); 
        }   
    };

    // run our function on load
    sticky_navigation();

    // and run it again every time you scroll
    $(window).scroll(function() {
         sticky_navigation();
    });

});
</script>

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现在脚本无法正常工作.我包括这样的脚本

<head><script src="http://ajax.googleapis.com/ajax/libs/jquery/1.6.4/jquery.min.js"></script>
<script type="text/javascript"  src="jful.js"></script></head>

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当我再次将脚本直接添加到HTML文档时,它可以工作!(当我们向下滚动页面时执行此脚本)问题是什么？