Using typeof method to judge undefined types

About JS to determine the type of undefined, using the TypeOf method, typeof return is a string, of which there is a undefined.

JS Judge undefined type

1if (revalue== undefined)

2{

3alert ("undefined");

4}

Found that the judge does not come out, finally checked the data to use the TypeOf method:

1if (typeof (revalue) = = "undefined")

2{

3alert ("undefined");

4}

typeof returns a string of six possible: "Number", "string", "Boolean", "Object", "function", "undefined".

Be sure to pay attention when you use it.