How to check if property exists on potentially undefined object? [duplicate]

Question

So there are questions on S.O. that answer how to check if a property on an object exists. Example Answer is to use Object.prototype.hasOwnProperty().

However, how would you check the property of a potentially undefined object?

If one were to attempt to just directly check if a property exists on an undefined object then it would be a reference error.

Semantically, it seems better to check code directly if (obj.prop) //dosomething -- it shows clearer intent. Is there any way to achieve this? Preferably, is there a built-in Javascript method to do such a thing or by convention?

Motive: A package adds property user.session.email -- but I'm checking to see if email exists, not the session, though the session could be nonexistent.

Update: Many of the answers say to use the && operator to short-circuit. I'm aware that this is a possible solution, but it is not exactly what is desired as it seems like we're working AROUND the JS object syntax -- that is, though you want to really check for a property on an object, you are being forced to check if the object exists in order to do so.

Note to being marked as closed Why was this marked as closed? The link that presumes this is a duplicate may not yield the same answers. We're looking for a better, more semantic solution and marking this closed makes the assumption that "nested" === "potentially undefined".

This question is disputably closed: Since I can't answer my own question now. As of recently, a proposal was put in Stage 1 here that would solve this very issue. Again, the question wants a more semantic solution as is shown in the proposal.

It wouldn’t be a syntactical error, it would be a ReferenceError. You can check obj && obj.prop. — Sebastian Simon
– Sebastian Simon, Commented Aug 25, 2016 at 2:19
Is the obj variable definitely going to exist but just might have an undefined value? By the way, testing if (obj.prop) doesn't test if a property exists, it tests if a property exists and has a truthy value. — nnnnnn
– nnnnnn, Commented Aug 25, 2016 at 2:19
Ah - fixed question description, but the question still remains valid because you're essentially checking to see if both an object and property exist, when I'm only trying to really check if a property exists. — steviesh
– steviesh, Commented Aug 25, 2016 at 2:20
@pocket Part of defensive programming is accounting for situations that will throw an error. If the object may not exist, you need to make sure it exists before referencing a property of it. — Jecoms
– Jecoms, Commented Aug 25, 2016 at 2:22
@nnnnnn admittedly, I have not seen this link before, and I think it's tangentially related and adds to the question - but the answers are not the same. Perhaps I can add detail to the question that I'm specifically looking for JS convention, or built-in ways to write clearer code. — steviesh
– steviesh, Commented Aug 25, 2016 at 2:34

Seth · Accepted Answer · 2016-08-25 02:37:47Z

If you're certain the parent object exists, then:

if(obj && obj.key) { console.log(obj.key); }

However, this example will fail miserably if obj doesn't exist.

if (obj && obj.key) { console.log(obj.key); }

... which is why you might want to check if the object is defined when checking for the property. This ensures that you won't get runtime errors if the condition is hit and the property's object hasn't been set.

if(typeof obj != "undefined" && obj.key) { console.log('you will not see this logged') }

You'll notice that the above example does not throw an exception.

var obj = {key: 1}; if(typeof obj != "undefined" && obj.key) { console.log('we have obj! and val is : ' + obj.key) }

This is a much more defensive approach but it allows for portability. If you wanted to extract your logic into a module for reuse, you may want to check if the object exists as well or risk unwanted errors from an area of the application which you might not necessarily be concerned with.

If there are no clearer ways to check the property, I will accept this as an answer (as submitted by other people).
Note that typeof obj != "undefined" will give a reference error if obj has been defined with let or const at a point after the typeof check.
Note that var a = { a:false, b:0, c:null} breaks these tests, since a.a and a.b and a.c all evaluate to not truthy, so if (a && a.a) { ... } will not think it's defined, but it is.

Nishanth Matha · Accepted Answer · 2018-02-19 00:51:07Z

if you want to check both obj and prop exists (like you mentioned in your comments) you have to do:

if(obj && obj.hasOwnProperty('some')) or if(obj && obj.some)

As mentioned in comments this will throw error if obj is undefined. A better comparison would be would be:

if(typeof obj != "undefined" && obj.hasOwnProperty('some'))

adonike · Accepted Answer · 2016-08-25 02:31:14Z

You could do something like:

if(user.session && 'email' in user.session) //do something...

in case you're sure that user is defined.

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How to check if property exists on potentially undefined object? [duplicate]

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