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Undefined vs Null in JavaScript

francescoxx profile image Francesco Ciulla ・2 min read

Maybe you don't need to read all the article, i just can show you this:

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undefined

undefined is a property of the global object.

It is a primitive value: undefined.

It is treated as falsy in boolean expressions.

undefined can be:

  • the type of a variable that has not been assigned yet.
  • the return value of a method or statement if the evaluated variable does not have an assigned value.
  • the return value of a function, if no value was returned.

You can also explicitly set a variable to undefined: (don't do it)

const a = undefined; //accepted, but can lead to confusion!
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null

null is an intentional absence of any object value.

It is a primitive value: null.

It is treated as falsy for boolean operations.

The value null is written with a literal: null.

null is not an identifier for a property of the global object.

Null expresses a lack of identification, indicating that a variable points to no object.

For Example, In many API, null is often retrieved in a place where an object can be expected but no object is relevant.

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

Also, undefined and null are two distinct types:

Unassigned variables are initialized by JavaScript with a default value of undefined.

JavaScript never sets a value to null, that must be done by the developer.

What do we get if we compare null and undefined with '==' and '===' operators?

console.log(null == undefined); // true
console.log(null === undefined); // false
console.log(typeof null); // "object" (not "null" for legacy reasons)
console.log(typeof undefined); // "undefined"
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Arithmetic Operations

Another difference is when we try to perform the arithmetic operation +

  • with null results as an integer
  • with undefined results is NaN
console.log(3 + null); //3
console.log(3 + undefined); //NaN
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in Conclusion

undefined typically means a variable has been declared, but not defined.

null is an assigned value, It means no value on purpose.

Discussion (5)

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linehammer profile image
linehammer

The value 'undefined' denotes that a variable has been declared, but hasn't been assigned any value. On the other hand, Javascript null refers to a non-existent object, which basically means 'empty' or 'nothing'. They're both values usually used to indicate the absence of something . Also, undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object .

net-informations.com/js/iq/nuvsun.htm

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drowvoloper profile image
Logan

I think telling null is an object is confusing. Null is a primitive value and the only reason it pretends to be an object is a bug.

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francescoxx profile image
Francesco Ciulla Author

yes, it is just the return value of typeof(null), an old well known bug. Thanks for noticing it Logan, I will see if it can be said in a better way

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drowvoloper profile image
Logan

That link helps, thank you!

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nickytonline profile image
Nick Taylor (he/him) • Edited

I think you'd both find this conversation interesting. Sinder makes some valid points for eradicating null even if some native APIs return null.

It's highly unlikely that null will be leaving the language, but regardless, there are still interesting convos in that GitHub issue.