Checking if undefined in jQuery/Javascript

Question

I have the following:

var currentQuestion = $(this).closest(".question")

I have tried everything suggested in this, this and this question:

if(currentQuestion !== undefined)
if(currentQuestion !== "undefined")
if(typeof currentQuestion !== undefined)
if(typeof currentQuestion !== "undefined")
if(currentQuestion !== null)
if(currentQuestion != undefined)
if(currentQuestion != "undefined")
if(currentQuestion.data("index") !== null)
if(currentQuestion.data("index") !== undefined)
if(typeof currentQuestion.data("index") !== undefined)

But it keeps going inside the if statement...

I have this inside the if:

console.log("nextQ: " + currentQuestion.data("index"));

and nextQ: undefined is getting print out

any other ideas?

EDIT:

currentQuestion.data("index") != null

worked out. If you check all the options I tried before, the one similar to this one had this comparison element: !== and not !=. That change made the difference. If someone can explain why, I'll grant him/her the correct answer.

console.log(currentQuestion.data("index"), currentQuestion); PS: just a side note: real computer science engineers don't make random things and expect them to work. — zerkms
– zerkms, Commented Jan 6, 2014 at 3:11
@zerkms i get the following: undefined [div.send-button, prevObject: b.fn.b.init[1], context: button.next, jquery: "1.9.1", constructor: function, init: function…] 0: div.send-button context: button.next length: 1 prevObject: b.fn.b.init[1] proto: Object[0] — marimaf
– marimaf, Commented Jan 6, 2014 at 3:12
@marimaf: now you see why your comparison to random undefined values isn't true - because it is defined and stores an object. — zerkms
– zerkms, Commented Jan 6, 2014 at 3:13
The reason that !== null doesn't work but != null does, is that the value returned is not null. When there is no data associated with the element, the data method returns undefined. Comparing that to null using the !== operator will give true, as null and undefined is not the same type. Comparing them using the != operator will give false as they are converted to a type that can be compared value to value, and the conversion will make them end up as the same value. To check if there is no data associated with the element, check the type that the data method returns. — Guffa
– Guffa, Commented Jan 6, 2014 at 21:20

Guffa · Accepted Answer · 2014-01-06 21:21:51Z

3

The result will never be undefined, it's always a jQuery object. If the operation didn't find any elements, the jQuery object will be empty, so you check how many element there are in it to see if found anything:

if (currentQuestion.length > 0)

After you have checked that there is actually any element in the jQuery object, you can check if there is any data associated to the element.

If no data is associated with an element, the data method will return undefined when you try to read the value. So, to check if there is no data, you should check the type of the value that the data method returns:

if (typeof currentQuestion.data("index") != "undefined")

edited Jan 6, 2014 at 21:21

answered Jan 6, 2014 at 3:13

Guffa

703k111 gold badges760 silver badges1k bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

marimaf Over a year ago

It still goes in with this solution

Qantas 94 Heavy Over a year ago

@marimaf: then there is something else going on that you haven't told us about.

marimaf Over a year ago

mm I'm receiving currentQuestion as a parameter in a function. Can that be the problem? I don't know what else

marimaf Over a year ago

currentQuestion gets updated with currentQuestion.next()

xdazz · Accepted Answer · 2014-01-06 03:11:58Z

1

If you want to check any elements exist, then check the length.

var currentQuestion = $(this).closest(".question");
if (currentQuestion.length > 0) {
    console.log("nextQ: " + currentQuestion.data("index"));
}

answered Jan 6, 2014 at 3:11

xdazz

161k38 gold badges255 silver badges278 bronze badges

2 Comments

bozdoz Over a year ago

Perhaps you don't have a data attribute on that selector @marimaf. Maybe what you can check is if (currentQuestion.data('index'))

marimaf Over a year ago

I have the data attribute. My code is moving from one question to the other in the same document and I need to know when there are no more questions to show. I am getting nextQ: 1, nextQ: 2, etc before the undefined

Damien Black · Accepted Answer · 2014-01-06 03:12:46Z

1

if (currentQuestion.length) {

Should work fine. If it goes in there, it found something. And instead of looking at the if statement you need to look at your html and see what it found.

answered Jan 6, 2014 at 3:12

Damien Black

5,65720 silver badges25 bronze badges

3 Comments

jammykam Over a year ago

Which suggests it found a match. Try console.log(currentQuestion) to see what it matched.

marimaf Over a year ago

I get this printed out: [div.send-button, prevObject: b.fn.b.init[1], context: button.next, jquery: "1.9.1", constructor: function, init: function…] 0: div.send-button context: button.next length: 1 prevObject: b.fn.b.init[1] proto: Object[0]

jammykam Over a year ago

In Chrome, You should be able to expand that and jump to the matched element. I'm not sure closest on it's own is correct, I would also expect a next selector, but without a jsFiddle of your html I doubt anyone else will be able to figure this out for you.

Community · Accepted Answer · 2017-05-23 12:03:44Z

1

What you want is currentQuestion.length.

jQuery selectors return an array of elements matching the selector. To test for values in an array, you should use length:

Boolean([].length); //false

Because 0 evaluates to false, you can just use if (currentQuestion.length).

If you're trying to check for it when it is false, use !:

if (!currentQuestion.length)

For your question of why != worked but not !==, I would suggest this question: Difference between == and === in JavaScript

Basically, currentQuestion.data('index') is not strictly equal to null, but it could evaluate to null: same as [] == 0 evaluates to true, but [] === 0 evaluates to false.

edited May 23, 2017 at 12:03

CommunityBot

11 silver badge

answered Jan 6, 2014 at 3:11

bozdoz

12.9k8 gold badges74 silver badges100 bronze badges

4 Comments

marimaf Over a year ago

I get length of 1... I tried currentQuestion.data("index").length but then I get an error I cannot call length of undefined...

bozdoz Over a year ago

Because the element exists but it doesn't have the data attribute. @marimaf Either check for currentQuestion.length or currentQuestion.data('index')

bozdoz Over a year ago

I read in a comment that you might be returning an object? If that's true you could do typeof(currentQuestion.data('index')) !== 'undefined'

marimaf Over a year ago

This worked after all: currentQuestion.data("index") != null what I tried before was !== instead of !=. Thanks for your help!

marimaf · Accepted Answer · 2014-01-06 19:10:21Z

0

It is probably not the most elegant solution. But somehow

currentQuestion.data("index") != null

worked out. If you check all the options I tried before, the most similar to this one had this comparison element: !== and not !=. That change made the difference. If someone can explain why, I'll grant him/her the correct answer.

answered Jan 6, 2014 at 19:10

marimaf

5,4203 gold badges53 silver badges70 bronze badges

Collectives™ on Stack Overflow

Checking if undefined in jQuery/Javascript

5 Answers 5

4 Comments

2 Comments

3 Comments

4 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

4 Comments

2 Comments

3 Comments

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related