<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<script src="https://cdn.bootcss.com/jquery/2.2.2/jquery.slim.js"></script>
</head>
<body>
<input id="test" name="test" type="text" />
<div class="box"></div>
</body>
<script type="text/javascript">
$(
'#test').
keyup(
function() {
var value2 =
$(
'#test').
val();
/*split(" ")先以空格分割为数组，join("")添加为新的字符串
* 判断新的字符串长度
*/
value2 =
value2.
split(
" ").
join(
"");
if(
value2.
length ==
0){
$(
'.box').
text(
"输入的值为空");
}
})
</script>
</html>
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