Introduction

For many javascript developers, null and undefined might look very simple but there might be some questions in your mind about null and undefined so, here is a blog explaining all the frequently asked questions about null and undefined.

Null

Null is a value which is known to exist but has no valid value has set.

Why null's value is zero?

An interesting fact about null is that, it's value is 0 when performing arithmetic operations,

console.log(null + 5); // 5

Some beginners confuse the value of null with 0 but is it really? No, null is not 0, its value becomes 0 when performing calculations.

console.log(+null); // 0

This is because when javascript converts null to 0 as stated in Annotated ES5 null is considered as 0 when converted into number while undefined is converted into NaN.

Why type of null is object?

So, another weirdest part of javascript is that null's type is object although it is a primitive.

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

Is there any logical explanation behind this? Well, it is a mistake done in the implementation of early Javascript.

Okay, it is a bug, then why it is not fixed yet? The reason for this is obvious, many old websites which run on old javascript might will face serious errors if typeof null becomes null instead of object and even if the bug is fixed, browser vendors have to implement this specification (Chrome's V8 engine, Firefox's SpiderMonkey, etc).

So, if you want to detect a null, you can just use a type strict equal operator.

const isNull = n => n === null;

console.log(isNull(0)); // false

Why null == undefined?

Another weird thing thing about null and undefined is that null equals undefined in javascript,

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

Why this now? According to the Javascript Language Specification, if the operands of the equal operator are null and undefined, the result would be false. And why is it? I was not able to find any valid reason for this but if you know a valid reason, you can drop your comment so others can know about it.

But, no worries! You can simply use the type strict operator (===) for this,

console.log(null === undefined); // false

Important Notes about Null

• Null is a falsy value so Boolean(null) == false.
• null is a reserved keyword

Undefined

Undefined is a value which means the value is not known to exist so, the type of undefined is undefined.

let x;

console.log(x); // undefined
console.log(typeof x); // "undefined"

Important Notes about Undefined

• Undefined is a falsy value like null so Boolean(undefined) == false.
• While stringifying undefined into JSON, it is converted into null, therefore, JSON.strinigify(undefined) == "null".
• undefined is not a reserved keyword so you can define a variable with the name undefined but not in global scope.
• Unlike null, using undefined for arithmetic operations would result in NaN, since, isNaN(Number(undefined)) == true.

Null vs Undefined

Another question frequently asked, what is the difference between null and undefined?

Understanding with example

Here are examples how to understand the usage of null and undefined.

Here, a non nullish value has returned while accessing the owner property from DogA which means the owner of DogA exists,

const DogA = { name: 'milo', owner: 'sudarsan' };
console.log(DogA.owner); // 'sudarsan'

Here, null is returned while accessing the owner property from DogB which means DogB can have an owner but has no owner.

const DogB = { name: 'charlie', owner: null };
console.log(DogB.owner); // null

Here, undefined is returned while accessing the owner property from DogC which means DogC cannot have an owner.

const DogC = { name: 'teddy' };
console.log(DogC.owner); // undefined

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The End

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Thank you for reading till the end!