button 按钮，结合onclick事件，验证和提交表单

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  <span style="background-color: #f5f5f5; color: #008000">//</span><span style="background-color: #f5f5f5; color: #008000">button 按钮，结合onclick事件，验证和提交表单</span>
  <span style="background-color: #f5f5f5; color: #0000ff">function</span><span style="background-color: #f5f5f5; color: #000000"> checkForm(){
        </span><span style="background-color: #f5f5f5; color: #008000">//</span><span style="background-color: #f5f5f5; color: #008000">判断用户名是否为空</span>
        <span style="background-color: #f5f5f5; color: #0000ff">if</span><span style="background-color: #f5f5f5; color: #000000">(document.form1.username.value</span><span style="background-color: #f5f5f5; color: #000000">==</span><span style="background-color: #f5f5f5; color: #000000">""</span><span style="background-color: #f5f5f5; color: #000000">){
                window.alert(</span><span style="background-color: #f5f5f5; color: #000000">"</span><span style="background-color: #f5f5f5; color: #000000">用户名不能为空</span><span style="background-color: #f5f5f5; color: #000000">"</span><span style="background-color: #f5f5f5; color: #000000">);
        }</span><span style="background-color: #f5f5f5; color: #0000ff">else</span> <span style="background-color: #f5f5f5; color: #0000ff">if</span><span style="background-color: #f5f5f5; color: #000000">(document.form1.username.value.length</span><span style="background-color: #f5f5f5; color: #000000"><span style="background-color: #f5f5f5; color: #000000">5</span> <span style="background-color: #f5f5f5; color: #000000">||</span><span style="background-color: #f5f5f5; color: #000000"> document.form1.username.value.length</span><span style="background-color: #f5f5f5; color: #000000">></span><span style="background-color: #f5f5f5; color: #000000">20</span><span style="background-color: #f5f5f5; color: #000000">){
            window.alert(</span><span style="background-color: #f5f5f5; color: #000000">"</span><span style="background-color: #f5f5f5; color: #000000">用户名长度必须介于5-50个字符之间！</span><span style="background-color: #f5f5f5; color: #000000">"</span><span style="background-color: #f5f5f5; color: #000000">);
        }</span><span style="background-color: #f5f5f5; color: #0000ff">else</span><span style="background-color: #f5f5f5; color: #000000">{
            </span><span style="background-color: #f5f5f5; color: #008000">//</span><span style="background-color: #f5f5f5; color: #008000">使用form对象的submit()方法，实现提交。</span>
<span style="background-color: #f5f5f5; color: #000000">            document.form1.submit();        
        }
  }
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  <span style="color: #0000ff"><span style="color: #800000">form </span><span style="color: #ff0000">name</span><span style="color: #0000ff">="form1"</span><span style="color: #ff0000"> method</span><span style="color: #0000ff">="get"</span><span style="color: #ff0000"> action</span><span style="color: #0000ff">="login.php"</span><span style="color: #0000ff">></span><span style="color: #000000">
  用户名：</span><span style="color: #0000ff"><span style="color: #800000">input </span><span style="color: #ff0000">type</span><span style="color: #0000ff">="text"</span><span style="color: #ff0000"> name</span><span style="color: #0000ff">="username"</span> <span style="color: #0000ff">/></span><span style="color: #000000">
  密码：</span><span style="color: #0000ff"><span style="color: #800000">input </span><span style="color: #ff0000">type</span><span style="color: #0000ff">="password"</span><span style="color: #ff0000"> name</span><span style="color: #0000ff">="userpwd"</span> <span style="color: #0000ff">/></span>
  <span style="color: #0000ff"><span style="color: #800000">input </span><span style="color: #ff0000">type</span><span style="color: #0000ff">="button"</span><span style="color: #ff0000"> value</span><span style="color: #0000ff">="提交表单"</span><span style="color: #ff0000"> onclick</span><span style="color: #0000ff">="checkForm()"</span> <span style="color: #0000ff">/></span>
  <span style="color: #0000ff"></span><span style="color: #800000">form</span><span style="color: #0000ff">></span>
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