The Undefined type has only one value, undefined.
When a variable is declared and not initialized, it is given the value of undefined by default.
var oTemp;
alert(oTemp == undefined);
You can also use the typeof operator to show that the variable has a value of undefined.
var oTemp;
alert(typeof oTemp);
A variable having the value of undefined is different from a value being undefined.
The typeof operator doesn't distinguish between the two.
var oTemp;
alert(typeof oTemp); //outputs "undefined"
alert(typeof oTemp2); //outputs "undefined"
If you try to use oTemp2 with any operator other than typeof, it causes an error.
alert(oTemp2 == undefined); //causes error
The value undefined is also returned when a function doesn't explicitly return a value:
function testFunc() {
//leave the function blank
}
alert(testFunc() == undefined); //outputs "true"