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来源：4-3 编程练习

鲨鱼没有鲨鱼线

2019-12-01 01:33:24

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>focus blur</title>
</head>

<body>
    <input type="text" value=""><span></span>
    <script src="https://cdn.bootcss.com/jquery/3.3.1/jquery.js"></script>
    <script>
    //此处写代码
        $(document).ready(function () {
            $('input').focus(function () {
                $('span')
                .text('请输入用户名')
                .css({'color':'#00ff00'})
            })
            .blur(function () {
                if($(this).val() == ""){
                    $('span')
                    .text('用户名不能为空')
                    .css({'color':'#ff0000'})
                } else{
                    $('span').text('')
                }
            })
        })
    </script>
</body>
</html>

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