JS数值变量和字符串变量

<script type="text/javascript">
        window.alert("你好");
        document.write("你好");
        document.write("<hr>");
        var a = "10";//所谓链接就是将左右字符串连接起来
        var b = 10;
        var c = a+b;
        document.write(c);
        document.write("<hr>");
        var name = "贾光圣";
        var xuehao  = "1201010739";
        var sex = "男";
        var adress = "成都";
        var str = ‘‘
            str +=    ‘<table width="500" border="1" align="center">‘;
            str +=    ‘    <tr>‘;
            str +=    ‘    <th colspan="6" align="center">西南石油大学学生信息表</th>‘;
            str +=    ‘    </tr>‘;
            str +=    ‘    <tr>‘;
            str +=    ‘        <td>姓名：</td>‘;
            str +=    ‘        <td>学号：</td>‘;
            str +=    ‘        <td>性别：</td>‘;
            str +=    ‘        <td>生源地：</td>‘;
            str +=    ‘    </tr>‘;
            str +=    ‘    <tr>‘;
            str +=    ‘        <td>‘+name+‘</td>‘;
            str +=    ‘        <td>‘+xuehao+‘</td>‘;
            str +=    ‘        <td>‘+sex+‘</td>‘;
            str +=    ‘        <td>‘+adress+‘</td>‘;
            str +=    ‘    </tr>‘; 
            str +=    ‘</table>‘;
        document.write(str);

    </script>

<script type="text/javascript">
        window.alert("你好");
        document.write("你好");
        document.write("<hr>");
        var a = "10";//所谓链接就是将左右字符串连接起来
        var b = 10;
        var c = a+b;
        document.write(c);
        document.write("<hr>");
        var name = "贾光圣";
        var xuehao  = "1201010739";
        var sex = "男";
        var adress = "成都";
        var str = ‘‘
            str +=    ‘<table width="500" border="1" align="center">‘;
            str +=    ‘    <tr>‘;
            str +=    ‘    <th colspan="6" align="center">西南石油大学学生信息表</th>‘;
            str +=    ‘    </tr>‘;
            str +=    ‘    <tr>‘;
            str +=    ‘        <td>姓名：</td>‘;
            str +=    ‘        <td>学号：</td>‘;
            str +=    ‘        <td>性别：</td>‘;
            str +=    ‘        <td>生源地：</td>‘;
            str +=    ‘    </tr>‘;
            str +=    ‘    <tr>‘;
            str +=    ‘        <td>‘+name+‘</td>‘;
            str +=    ‘        <td>‘+xuehao+‘</td>‘;
            str +=    ‘        <td>‘+sex+‘</td>‘;
            str +=    ‘        <td>‘+adress+‘</td>‘;
            str +=    ‘    </tr>‘; 
            str +=    ‘</table>‘;
        document.write(str);

    </script>

<script type="text/javascript">
        window.alert("你好");
        document.write("你好");
        document.write("<hr>");
        var a = "10";//所谓链接就是将左右字符串连接起来
        var b = 10;
        var c = a+b;
        document.write(c);
        document.write("<hr>");
        var name = "贾光圣";
        var xuehao  = "1201010739";
        var sex = "男";
        var adress = "成都";
        var str = ‘‘
            str +=    ‘<table width="500" border="1" align="center">‘;
            str +=    ‘    <tr>‘;
            str +=    ‘    <th colspan="6" align="center">西南石油大学学生信息表</th>‘;
            str +=    ‘    </tr>‘;
            str +=    ‘    <tr>‘;
            str +=    ‘        <td>姓名：</td>‘;
            str +=    ‘        <td>学号：</td>‘;
            str +=    ‘        <td>性别：</td>‘;
            str +=    ‘        <td>生源地：</td>‘;
            str +=    ‘    </tr>‘;
            str +=    ‘    <tr>‘;
            str +=    ‘        <td>‘+name+‘</td>‘;
            str +=    ‘        <td>‘+xuehao+‘</td>‘;
            str +=    ‘        <td>‘+sex+‘</td>‘;
            str +=    ‘        <td>‘+adress+‘</td>‘;
            str +=    ‘    </tr>‘; 
            str +=    ‘</table>‘;
        document.write(str);

    </script>