JS judgment of undefined implementation code_javascript skills

The following is incorrect usage:

Copy code The code is as follows:

var exp = undefined;
if (exp == undefined)
{
alert("undefined");
}

When exp is null, the same result as undefined will be obtained , although null and undefined are not the same.
The following is the correct usage:

Copy the code The code is as follows:

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

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