javascript null vs undefined

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

1660

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

812

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.

MakeUseOf

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What's the Difference Between Null and Undefined in JavaScript?

June 15, 2021 - In JavaScript, null is a primitive value that is used to signify the intentional absence of an object value, whereas undefined is a primitive value that acts as a placeholder for a variable that has not been assigned a value.

Discussions

'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

9

4

June 7, 2023

How do null and undefined differ in JavaScript programming?

I’m working on a JavaScript project and I keep running into situations where I encounter both null and undefined values. I’m getting confused about when each one appears and what they actually mean. For example, when I create variables or work with object properties, sometimes I see one ... More on community.latenode.com

community.latenode.com

1

0

June 2, 2025

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

51

46

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

43

132

March 26, 2017

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