.data() getter creates data

[prontiol]

Not sure, if this is intended, but calling .data() without arguments creates data object for the given node.

> $.hasData($div)
false
> $($div).data()
Object {}
> $.hasData($div)
true

[dmethvin]

Yes, it's intended. The only way the method can give you a data object attached to the element is to make one if it's not there. That's why there's .hasData to let you check if there's one already.

[dmethvin]

Also, the docs mention this: http://api.jquery.com/jquery.hasdata/

[prontiol]

Well, docs for .hasData() also say:

If no data is associated with an element (there is no data object at all or the data object is empty), the method returns false;

But in the test case I provided .hasData() returns true for an empty object.

[gibson042]

Agreed; this is legitimate: https://jsfiddle.net/d6t7mm5w/

[dmethvin]

The code is behaving the way we'd want it to though, correct? The docs are wrong.

[prontiol]

But why getter does actually create data object? What's the reason?

[dmethvin]

Because the .data() with no args is defined to return the data object. If one isn't there, it creates one, attaches it to the element, and returns that new empty object.

Ten years ago, the API could have been defined to return null or undefined for an element with no data and you would have needed to set a new data item explicitly. That's not the way it was defined though, and lots of code no doubt depends on that behavior now. The assumption was that if you were trying to get the full data item you actually wanted to use it, and the current behavior makes that simpler. So jQuery.hasData() was created as a way to see if there was data on an element without creating it, the presumably less common case.

[gibson042]

Ok, @dmethvin is right:

Note that jQuery's event system uses the jQuery data API to store event handlers. Therefore, binding an event to an element using .on(), .bind(), .live(), .delegate(), or one of the shorthand event methods also associates a data object with that element.

In this case, it's hasDataAttrs private data. Which makes jQuery.hasData essentially useless, but that's a separate issue.

[dmethvin]

The goal of jQuery.hasData is to give the caller a way to avoid accidentally creating an unneeded data object and it's still useful for that. It doesn't matter whether it's private or public data, just whether there's already an object. I agree that it's useless for a user making inferences such as "Have I put some data on this object? Okay I'll get the object and enumerate it. Hey it's empty, that should never happen!"

[prontiol]

I think we need to fix the docs about jQuery.hasData saying it will return false for empty data objects, because it is just not true.

[dmethvin]

Agreed, just created jquery/api.jquery.com#889