js中提示input中为空的情况

Question

怎么用js做这种效果，求代码

Answer 1

 <style>
    input.error{border:solid 1px #f00}
    span.error{color:#f00;display:none}
</style>
<input type="text" onblur="this.className = this.value == '' ? 'error' : ''; this.nextSibling.style.display = this.value == '' ? 'inline' : 'none'" /><span class="error">不能为空！</span>