What is the difference between null and undefined in JavaScript?

Question

I want to know what the difference is between null and undefined in JavaScript.

Answer 1

null is a special keyword that indicates an absence of value.

think about it as a value, like:

• "foo" is string,
• true is boolean ,
• 1234 is number,
• null is undefined.

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undefined property indicates that a variable has not been assigned a value including null too . Like

var foo;

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defined empty variable is null of datatype undefined

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

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

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Like

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