前端判断null、undefined与NaN的方法_前端显示underfunded

https://www.cnblogs.com/pangguoming/p/7690816.html
undefined

var tmp = undefined; 
if (typeof(tmp) == "undefined"){ 
alert("undefined"); 
}
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null

var tmp = null; 
if (!tmp && typeof(tmp)!="undefined" && tmp!=0){ 
alert("null"); 
}
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NaN

var tmp = 0/0; 
if(isNaN(tmp)){ 
alert("NaN"); 
}
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