<input type="text" name="" id="results" value=""/>

var content = $('#content').val();
if(content.trim() == '')
alert('空');

<input type="text" name="" id="results" value=""/>

var content = $('#results').val();
if($.trim(content) == '')
alert('空');

var content = $('#content').val();
if(jQuery.trim(content) == '')
alert('空');