NULL vs UNDEFINED in JS

NULL

What is null?

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

There are two features of null you should understand:

• null is an empty or non-existent value.
• null must be assigned.

In the example below, we are assigning value to null.

var x = null
console.log(x);
// null 

• null expresses a lack of identification, indicating that variable points to no object
• When you assign null to a variable, you are declaring that this value is explicitly empty.

UNDEFINED

What is undefined?

Undefined usually means a variable has been declared, but not defined.

• Another common scenario, where you will encounter an undefined is while trying to access a property of an object that doesn’t exist.

var z = {};
console.log(z.value);
// undefined

• To summarize,
  • An object is declared but not defined/initialized.
  • Accessing an object property or an array index that does not exist.
  • Calling a function, without it’s required function parameters.

What is similar?

• Both nulland undefined are primitive values in JavaScript.
• Another thing to note is that when comparing null and undefined they are equal.

null == undefined
// true

• This is because both of them are considered falsy values in JavaScript.

Differences nullvs. undefined

• Arithmetic Operations

Another interesting difference to note is when arithmetic operations are performed with null vs. undefined.

Arithmetic operations with a null value will result in integer value while any arithmetic operation with undefined will result in the value of the variable being changed to NaN.

var x = 5 + null;
console.log(x);
// 5

var y = 5 + undefined;
console.log(y);
// NaN 

• Equality

• Typeof Null and Undefined

• Interestingly enough, when using typeof to test null, it returns an object:

console.log(typeof(undefined));  //"undefined"
console.log(typeof(null));       //"object"

• Comparison Using Abstract and Strict Equality

• Since these are different data types, if we compare them with strict equality ===, we get false.
• But if we compare them with abstract equality ==, we get true.

console.log(null === undefined)    //false
console.log(null == undefined)     //true
console.log(null !== undefined)    //true

Summary

• null is an assigned value. It means nothing.
• undefined typically means a variable has been declared but not defined yet.
• null and undefined are falsy values.
• null and undefined are both primitives. However, an error shows that typeof null = object.
• null !== undefined but null == undefined.