The Undefined Type : Introduction « Number Data Type « JavaScript Tutorial

1. JavaScript Tutorial
2. Number Data Type
3. Introduction

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"