null vs undefined in JavaScript

The concepts of NaN (Not-a-Number) and undefined in JavaScript are both special values, but they serve very different purposes. Below, you'll find a detailed comparison, a table summarizing the key differences, and an example to clarify these distinctions.

NaN vs undefined in JavaScript 

NaN (Not-a-Number)

The concepts of null and undefined in JavaScript are common but can be a source of confusion. Both represent the absence of a value, but they are used in slightly different circumstances. Here’s a detailed explanation along with a table and an example to illustrate the difference.

null vs undefined in JavaScript

null

null is an assignment value that represents no value or no object. It's a value you can assign to a variable when you want to intentionally indicate that the variable has no value.

undefined

undefined is a primitive value automatically assigned to variables that have just been declared or to function arguments for which there are no corresponding parameters. Essentially, it means that the variable exists but has not been assigned a value yet.

Comparison Table

Feature	'null'	'undefined'
Type	Object	Undefined
Value	Represents no value or no object	Indicates a variable hasn't been assigned a value
Explicit Assignment	Can be explicitly assigned	Usually assigned by JavaScript
Comparative with '=='	null == undefined is true	undefined == null is true
Comparative with '==='	null === undefined is false	undefined === null is false

Example

Let’s look at an example to better understand null and undefined.

var a;
var b = null;

console.log(a); // Outputs 'undefined' because 'a' is declared but not assigned a value
console.log(b); // Outputs 'null' because 'b' is explicitly assigned to 'null'

console.log(a == b);  // Outputs true because 'undefined' and 'null' are loosely equal
console.log(a === b); // Outputs false because 'undefined' and 'null' are not strictly equal

Conclusion

While both null and undefined represent an absence of value in JavaScript, they are used in different contexts. null is usually assigned explicitly to indicate "no value," while undefined is assigned by the system when a variable is declared but not assigned.

Understanding these differences is key to avoiding unexpected behavior in your code. While they may seem interchangeable, using them properly according to the context is essential for clean and predictable coding. If you want to check for both null and undefined, you can use the == operator, but if you want to distinguish between them, make sure to use the === operator.

NaN is a special numeric value that represents the result of an undefined or unrepresentable mathematical operation. For example, dividing 0 by 0 or taking the square root of a negative number will result in NaN. It is important to note that NaN is not equal to anything, even itself.

undefined

undefined is a primitive value automatically assigned to variables that have just been declared or to function arguments for which there are no corresponding parameters. It indicates that a variable exists but has not been assigned a value yet.

Comparison Table

Feature	'NaN'	'undefined'
Type	Number	Undefined
Value	Represents unrepresentable number	Indicates a variable hasn't been assigned a value
Usage	Result of invalid mathematical operations	Result of declaring without initializing
Comparative with '=='	NaN == NaN is false	undefined == undefined is true
Comparative with '==='	NaN === NaN is false	undefined === undefined is true

Example

Here’s an example to illustrate the concepts of NaN and undefined.

var a = 0/0; // Invalid division
var b;

console.log(a); // Outputs 'NaN' because 0 divided by 0 is unrepresentable
console.log(b); // Outputs 'undefined' because 'b' is declared but not assigned a value

console.log(a == a);  // Outputs false because NaN is not equal to itself
console.log(b == b);  // Outputs true because 'undefined' is equal to itself

console.log(a === a); // Outputs false because NaN is not strictly equal to itself
console.log(b === b); // Outputs true because 'undefined' is strictly equal to itself

Conclusion

NaN and undefined are both special values in JavaScript, but they represent entirely different concepts. NaN is used to indicate an unrepresentable mathematical result, while undefined is used to signify that a variable has been declared but not yet assigned a value.

Understanding these differences and when each value is likely to occur is crucial for debugging and maintaining clear, functional code. Using functions like isNaN() to check for NaN and being mindful of variable initialization can help prevent unexpected behaviors related to these values in your code.