How to determine if jquery is empty

Jquery method to determine whether it is empty: 1. Use undefined to determine, the code is [if(C==null||C==undefined||C==""){//code}]; 2. To determine the length, the code is [if(C.length==0){//code}].

The operating environment of this tutorial: windows7 system, jquery3.2.1 version, thinkpad t480 computer.

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##jquery method to determine whether it is empty:

if($("#email").val()) {
           var isEmail = /^([\.a-zA-Z0-9_-])+@([a-zA-Z0-9_-])+(\.[a-zA-Z0-9_-])+/;
           if(!isEmail.test($("#email").val())){
             $('#popContent').text('请您输入正确的邮箱');
             $('#popMask').modal('open');
             return false;
           }
}

var C = $("class name or id").val();

if(C==null||C==undefined||C==""){
//代码
}

if(C.length==0){
//代码
}

if(C){
//代码
}

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