js uses typeof method to determine undefined type_javascript skills

js determines undefined type

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

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

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

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