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Detecting an undefined object property

How do I check if an object property in JavaScript is undefined?

Answer*

What does this mean: **"undefined object property"**?

Actually it can mean two quite different things! First, it can mean *the property that has never been defined* in the object and, second, it can mean the *property that has an undefined value*. Let's look at this code:

 var o = { a: undefined }

Is `o.a` undefined? Yes! Its value is undefined. Is `o.b` undefined? Sure! There is no property 'b' at all! OK, see now how different approaches behave in both situations:

 typeof o.a == 'undefined' // true
 typeof o.b == 'undefined' // true
 o.a === undefined // true
 o.b === undefined // true
 'a' in o // true
 'b' in o // false

We can clearly see that `typeof obj.prop == 'undefined'` and `obj.prop === undefined` are equivalent, and they do not distinguish those different situations. And `'prop' in obj` can detect the situation when a property hasn't been defined at all and doesn't pay attention to the property value which may be undefined.

So what to do?
--------------

1) You want to know if a property is undefined by either the first or second meaning (the most typical situation).

 obj.prop === undefined // IMHO, see "final fight" below

2) You want to just know if object has some property and don't care about its value.

 'prop' in obj


Notes:
------
 - You can't check an object and its property at the same time. For example, this `x.a === undefined` or this `typeof x.a == 'undefined'` raises `ReferenceError: x is not defined` if x is not defined.
 - Variable `undefined` is a global variable (so actually it is `window.undefined` in browsers). It has been supported since ECMAScript 1st Edition and since ECMAScript 5 it is **read only**. So in modern browsers it can't be *redefined to true* as many authors love to frighten us with, but this is still a true for older browsers.

Final fight: `obj.prop === undefined` vs `typeof obj.prop == 'undefined'`
-------------------------------------------------------------------------
Pluses of `obj.prop === undefined`:

 - It's a bit shorter and looks a bit prettier
 - The JavaScript engine will give you an error if you have misspelled `undefined`

Minuses of `obj.prop === undefined`:

 - `undefined` can be overridden in old browsers

Pluses of `typeof obj.prop == 'undefined'`:

 - It is really universal! It works in new and old browsers.

Minuses of `typeof obj.prop == 'undefined'`:

 - `'undefned'` (*misspelled*) here is just a string constant, so the JavaScript engine can't help you if you have misspelled it like I just did.

Update (for server-side JavaScript):
----------------------------
Node.js supports the global variable `undefined` as `global.undefined` (it can also be used without the 'global' prefix). I don't know about other implementations of server-side JavaScript.

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@Bergi thank you for your comment. I have corrected my answer. In my defense I can say that currently (as of v.0.10.18) official Node.js documentation says nothing about undefined as a member of global. Also neither console.log(global); nor for (var key in global) { ... } doesn't show undefined as a member of global. But test like 'undefined' in global show the opposite.

Konstantin Smolyanin
– Konstantin Smolyanin

2013年09月11日 10:53:06 +00:00
Commented Sep 11, 2013 at 10:53
4

It didn't need extra documentation since it's in the EcmaScript spec, which also says that [[Enumerable]] is false :-)

Bergi
– Bergi

2013年09月11日 11:00:10 +00:00
Commented Sep 11, 2013 at 11:00
5

Regarding Minuses of typeof obj.prop == 'undefined', this can be avoided by writing as typeof obj.prop == typeof undefined. This also gives a very nice symmetry.

hlovdal
– hlovdal

2014年10月24日 11:01:21 +00:00
Commented Oct 24, 2014 at 11:01
3

@hlovdal: That’s totally pointless vs. obj.prop === undefined.

Ry-
– Ry- ♦

2018年04月11日 21:38:27 +00:00
Commented Apr 11, 2018 at 21:38
2

When we are true to the question headline „Detecting an undefined property", not true to the (different and much easier) question in the first sentence („check if undefined..."), you answer if ('foo' in o)... your answer is truly the first correct answer here. Pretty much everybody else just answers that sentence.

Frank N
– Frank N

2018年06月11日 08:46:50 +00:00
Commented Jun 11, 2018 at 8:46

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Detecting an undefined object property

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