判断某个元素是否存在:
if($('#username').length>0){
alert('1');
}
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判断某个变量是否为undefined:
var a;
if(typeof(a)=="undefined"){
alert('1');
}
判断某个元素是否存在:
if($('#username').length>0){
alert('1');
}
______________________________________________________________________________________________________
判断某个变量是否为undefined:
var a;
if(typeof(a)=="undefined"){
alert('1');
}