Friday, November 15, 2013

undefined and null in javascript. What is the difference ?

There is one difference between undefined And null in JavaScript undefined is a data type and null is a value.


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


Both of them are representing a value of a variable with no value

AND null doesn't represent a string that has no value

Like

var a = ''; 
console.log(typeof a); // string 

var a = ''  ;
console.log(a == null); //false 
console.log(a == undefined); // false 

Now if

var a ;
console.log(a == null); //true
console.log(a == undefined); //true 

But

var a; 
console.log(a === null); //false 
console.log(a === undefined); // true

So each one has it own way to use

undefined use it to compare the variable data type

null use it to empty a value of a variable

var a = 'javascript';
a = null ; // will change the type of variable "a" from string to undefined 

Now it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.

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