undefined vs null javascript

What is the difference between null and undefined in JavaScript?

stackoverflow.com › questions › 5076944 › what-is-the-difference-between-null-and-undefined-in-javascript

undefined means a variable has been declared but has not yet been assigned a value :

var testVar;
console.log(testVar); //shows undefined
console.log(typeof testVar); //shows undefined

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

var testVar = null;
console.log(testVar); //shows null
console.log(typeof testVar); //shows object

From the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.

Proof :

console.log(null === undefined) // false (not the same type)
console.log(null == undefined) // true (but the "same value")
console.log(null === null) // true (both type and value are the same)

and

null = 'value' // Uncaught SyntaxError: invalid assignment left-hand side
undefined = 'value' // 'value'

Answer from sebastian on Stack Overflow

Stack Overflow

stackoverflow.com › questions › 5076944 › what-is-the-difference-between-null-and-undefined-in-javascript

What is the difference between null and undefined in JavaScript? - Stack Overflow

Top answer

1 of 16

1651

undefined means a variable has been declared but has not yet been assigned a value :

var testVar;
console.log(testVar); //shows undefined
console.log(typeof testVar); //shows undefined

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

var testVar = null;
console.log(testVar); //shows null
console.log(typeof testVar); //shows object

From the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.

Proof :

console.log(null === undefined) // false (not the same type)
console.log(null == undefined) // true (but the "same value")
console.log(null === null) // true (both type and value are the same)

and

null = 'value' // Uncaught SyntaxError: invalid assignment left-hand side
undefined = 'value' // 'value'

2 of 16

803

The difference can be explained with toilet tissue holder:

A non-zero value is like a holder with roll of toilet tissue and there's tissue still on the tube.
A zero value is like a holder with an empty toilet tissue tube.
A null value is like a holder that doesn't even have a tissue tube.
An undefined value is similar to the holder itself being missing.

GeeksforGeeks

geeksforgeeks.org › javascript › undefined-vs-null-in-javascript

Undefined Vs Null in JavaScript - GeeksforGeeks

July 23, 2025 - undefined indicates a variable hasn’t been initialized, while null is intentionally assigned to indicate no value. Understanding the distinction helps write cleaner, more predictable code in JavaScript, especially when handling default values or checking for missing data.

Discussions

Undefined vs null

Hello. What is the difference between undefined and null? More on forum.freecodecamp.org

forum.freecodecamp.org

November 19, 2019

Undefined vs null

One reason to favor undefined over null is how javascript handle default values: const withDefault = (a = true) => { console.log(a); }; withDefault(); // logs true withDefault(undefined); // logs true withDefault(null); // logs null More on reddit.com

r/typescript

February 27, 2023

null vs undefined

I try to think of it simply as: undefined means exactly that, we have not bothered to set a value for this thing. null means we explicitly set a value and wanted it to be anomalous. null shows intentionality you cannot infer from undefined. More on reddit.com

r/javascript

132

June 21, 2017

'null' or 'undefined': What should I use if I want to clear the variable from the memory?

As NateDzMtz says, the memory considerations are the same. null and undefined are unique values and don't involve any references into the heap. In this regard, false would have the same effect. As far as which is convenient for programming, since indexing an object with a key that is not found in the object returns undefined, storing undefined as the value almost simulates absence of the key. Of course, a query can be made to distinguish the case that foo has no key bar from the case where it has the key bar but undefined is stored as the value at that key. But if your design is such that those cases don't have different meanings, it's convenient to stifle slots by putting undefined in them. Note that delete can be inefficient in some engines and they are not required by the standard to make it efficient. I think that the conventional meanings of the special values are, more or less: undefined -- maybe was never initialized; isn't associated to any particular data type. null -- no object, where an object might be expected. NaN -- no number, where a number might be expected. false -- just not true, no other meaning. Note that typeof null is "object", even though you can't index null. typeof undefined is "undefined". typeof NaN is "number", even though NaN explicitly and exactly means "Not a Number"! More on reddit.com

r/learnjavascript

June 7, 2023

Videos

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January 23, 2024

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