<script>

//输出方式

document.write(Date());//获取当前时间

document.write(1);

document.write("<p>你好</p>");//直接输内容

document.getElementById("nihao").innerHTML;//通过获取id

document.getElementById("nihao").innerHTML="走";

//alert("你好");//alert输出

</script>

<form id="f1" name="f1" onsubmit="return checkform()">

用户名：<input type="text" name="username" value="" />

<input type="submit" value="提交" />

</form> 

<script>

function checkform()

{

    var f11=document.f1.username.value.length;

    if(f11>4)

    {alert("小于4");

     return false;}

    }

</script>

<form onsubmit="return validation()">

    <span>账号</span><input type="text" id="no" /><br>

    <span>密码</span><input type="text" id="pwd" /><br>

    <button type="submit">提交</button>

</form>

<script>

function validation() {

    var no = document.getElementById("no").value;

    var pwd = document.getElementById("pwd").value;

    if(no == "") {

        alert("账号不为空!");

        return false;

    }

if(pwd == "") {

        alert("密码不为空!");

        return false;

    }

}

</script>

<form name=a onsubmit="return test()">

<textarea name="b"  cols="40"  wrap="VIRTUAL" rows="6"></textarea>

<input type="submit" name="Submit" value="check">

</form>

<script>

function test()

{

if(document.a.b.value.length>10)

{

alert("不能超过10个字符！");

document.a.b.focus();

return false;

}

}

</script>