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https://www.cnblogs.com/pangguoming/p/7690816.html
undefined
var tmp = undefined;
if (typeof(tmp) == "undefined"){
alert("undefined");
}
null
var tmp = null;
if (!tmp && typeof(tmp)!="undefined" && tmp!=0){
alert("null");
}
NaN
var tmp = 0/0;
if(isNaN(tmp)){
alert("NaN");
}
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