Js determines the undefined type

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

It was found that the judgment could not be made, and the typeof method was used to check the data at last:

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

  Typeof returns a string, with six possibilities: “number”, “string”, “Boolean”, “object”, “function”, “undefined”