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Question: What is the difference between undefined value and null value?

Answer: (i)Undefined value cannot be explicitly stated that is there is no keyword called undefined whereas null value has keyword called null
(ii)typeof undefined variable or property returns undefined whereas
typeof null value returns object



Category Java Script Interview Questions & Answers - Exam Mode / Learning Mode
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Question: What is the difference between undefined value and null value?
Answer:

(i)Undefined value cannot be explicitly stated that is there is no keyword called undefined whereas null value has keyword called null
(ii)typeof undefined variable or property returns undefined whereas
typeof null value returns object Source: CoolInterview.com


In JavaScript, undefined means a variable has been declared but has not yet been assigned a value, such as:

var TestVar;
alert(TestVar); //shows undefined
alert(typeof TestVar); //shows undefined

null is an assignment value. It can be assigned to a variable as a representation of no value:

var TestVar = null;
alert(TestVar); //shows null
alert(typeof TestVar); //shows object

From the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.

Unassigned variables are initialized by JavaScript with a default value of undefined.

JavaScript never sets a value to null. That must be done programmatically. As such, null can be a useful debugging tool. If a variable is null, it was set in the program, not by JavaScript.

null values are evaluated as follows when used in these contexts:

Boolean: false
Numeric: 0
String: ?null?

undefined values are evaluated as follows when used in these contexts:

Boolean: false
Numeric: NaN
String: ?undefined? Source: CoolInterview.com

Answered by: SUSHILA | Date: 5/10/2010 | Contact SUSHILA Contact SUSHILA


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