javascript(jquery)函数返回值问题

$(document).ready(function(){
//提交按钮
$('#submit').click(function(){
var name=$('#username').val();
var psd=$("#password").val();
var cpsd=$("#checkpsd").val();
var age=$("#age").val();
var p=$('#telephone').val();
var u=$('#unit').val();
if(checkn(name) && checkpsd(psd) && checkcp(cpsd,psd) && checkage(age) && checkp(p) && checku(u))
{
$('#registerInfo').submit();
}
else
{
alert("请检查表单填写是否正确！");
}
});
//清空按钮
$('#reset').click(function(){
$('#nameTip').attr("class","non");
$('#nameTip').html("");
$('#psdTip').attr("class","non");
$('#psdTip').html("");
$('#cpsdTip').attr("class","non");
$('#cpsdTip').html("");
$('#ageTip').attr("class","non");
$('#ageTip').html("");
$('#pTip').attr("class","non");
$('#pTip').html("");
$('#uTip').attr("class","non");
$('#uTip').html("");
$('#unit').attr("class","validInput");
$('#telephone').attr("class","validInput");
$('#age').attr("class","validInputn");
$('#checkpsd').attr("class","validInput");
$('#password').attr("class","validInput");
$('#username').attr("class","validInput");
});
});
function checkn(name)
{
if(name=="")
{
return false;
}
else if(name.length<3 || name.length>15)
{
return false;
}
else
{
var a;
$.post('checkUserName',
{
username:name
}, function(data){
if(data=="false")
{
a=false;
}
else
{
a=true;
}
});
return a;
//这里a是显示true的，但是上面的checkn(name)的值是undefined，其他函数都可以正常返回true或者false
}
}

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在script中加上：
$.ajaxSetup({
async:false

})

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