undefined vs null in JavaScript

A variable is said to be “undefined” if it has been declared but not initialized. Whereas “null” is assigned to a variable whose value is absent at that moment.

In simple words, when you do not assign value to a variable, JavaScript engine treats it as undefined. The null value is assigned by programmer to indicate that variable has nothing inside of it, but intend to have value later in program execution.

undefined

See below JavaScript statements to understand the undefined.

var myVar;		//Variable declaration without assigning any value to it

console.log( myVar );	//undefined

console.log( typeof(myVar) );	//undefined

console.log( undeclaredVar );	//Uncaught ReferenceError: undeclaredVar is not defined

If you see a variable with value in undefined, then you know that:

1. The variable has been declared in the program.
2. There is no value assignment done for the variable.
3. If you check the type of variable, it will be undefined. Please note that is one of primitive types [string, number, boolean, null, undefined, symbol (new in ES6)] in JavaScript.
4. If a function returns undefined, it didn’t returned any value.

undefined is not reserved keyword in JavaScript.

null

null represents intentional absence of value. Conceptually, it’s very much like null in other programming languages like Java, C# etc. null expresses a lack of identification, indicating that a variable points to no object.

var myVar = null;		//Variable declared and assigned to null

console.log( myVar );	//null

console.log( typeof(myVar) );	//object

Please note that null is of type object with a valid value, no properties, is non-mutable, and only a single instance of the same exists in the system at all times.

Using equality operators with null and undefined

Always remember to use strict equality operator ( === ) for comparing variables with null and undefined, when you want to clearly distinguish between null and undefined variables.

Read More: == vs === in JavaScript

var myUndefinedVar;			//undefined
var myNullVar = null;		//null

myUndefinedVar == myNullVar;		//true - which not correct as both are different	

myUndefinedVar === myNullVar;		//false - correct behavior

null evaluates to zero

One major difference between null and undefined is in the way they are converted to primitive types.

• null is converted to zero (0) while performing primitive operations.
• undefined is converted to NaN.

var myUndefinedVar;			//undefined
var myNullVar = null;		//null

myUndefinedVar + 100;		//NaN

myNullVar + 100;			//100

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Happy Learning !!