问一下各位，如何根据input的状态显示div的内容

用js如何来写
“如果手机号码框为空，则在下面一行的div里显示'手机号码必填！'”
“如果手机号码框不为空，则在下面一行的div里什么也不显示”

推荐你使用 tooltips 插件，很方便，并且不影响布局，jquery tooltips 可以自行百度一下

Tooltip | jQuery UI jQuery UI is a curated set of user interface interactions, effects, widgets, and themes built on top of the jQuery JavaScript Library. Whether you're building highly interactive web applications or you just need to add a date picker to a form control, jQuery UI is the perfect choice. https://jqueryui.com/tooltip/