$("#btn").click(function(){ });只有在页面加载的时候才会有效触发

例：

<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title></title>
        <script type="text/javascript" src="js/jquery-2.0.0.js"></script>
        <script>
            var j = "";
            $(function(){
                j = 1;
                $("#btn").click(function(){ //函数在$(function(){})内
                    alert(j);
                });
            });
        </script>
    </head>
    <body>
        <input type="button" value="按钮" id="btn" />
    </body>
</html>

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