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

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JavaScript Tutorial » Number Data Type » Introduction 
5. 1. 8. The Undefined Type

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"
5. 1. Introduction
5. 1. 1. Numbers
5. 1. 2. Number() Object
5. 1. 3. Variables are loosely typed.
5. 1. 4. Built-in Values
5. 1. 5. Special Numerical Values
5. 1. 6. Primitive and Reference Values
5. 1. 7. Primitive Types
5. 1. 8. The Undefined Type
5. 1. 9. The Null Type
5. 1. 10. The Boolean Type
5. 1. 11. The Number Type
5. 1. 12. Number.MAX_VALUE and Number.MIN_VALUE define the outer bounds of the Number value set
5. 1. 13. NaN stands for Not a Number
5. 1. 14. JavaScript primitive values, Booleans, numbers, and strings, are pseudo-objects
5. 1. 15. Reference Types
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