js-undefined type judgment

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js judges undefined type

  if (reValue== undefined){
    alert("undefined");
    }

I found that I couldn’t tell, so I finally checked the information and needed to use typeof
method:

if (typeof(reValue) == "undefined") {
    alert("undefined"); 
    }

typeof Return is a string, there are six possibilities: "number", "string", "boolean", "object", "function", "undefined"

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