`context` issue (incompatible change) when using DOM elements with $

[T.J. Crowder]

As of 1.4.x, using a DOM element with $() apparently sets the jQuery object's context property, which changes the behavior of functions like add incompatibly with v1.3.x.

These two lines, which look like they should be identical, do very different things:

var a = $('#first').add('li');

var b = $(document.getElementById('first')).add('li');

The first line finds the element with the ID first and adds any lis in the document to it.

The second line finds the element with the ID first and adds only lis within that element.

Naturally this has a pretty big effect on event handlers, where $(this) (with this being a DOM element) is a very common use case.

Here's the ​the `add` example code adapted to demonstrate the problem ​using 1.4.4 (​1.4.0 also has the issue). ​1.3.2 works fine.

This behavior is very surprising. People will expect the two lines above to do the same thing. I've seen people saying those two uses of $() would be functionally the same all over the place, and of course they were the same prior to 1.4.0.

This looks and acts very much like a bug, but 1.4.0 came out nearly a year ago and the issue has existed from then through to 1.4.4 (the latest as of this writing), which makes me wonder if it's an intentional compatibility-breaking change. If so, it's unfortunate (IMHO, but I'm not involved with jQuery's development). It's also not mentioned in the ​release notes. In any case, if it remains, it needs substantial discussion in the documentation, not least the ​`jQuery()` documentation, ​`add` documentation, and anywhere else this.context will have this impact. But unless it was a very carefully considered change, strongly recommend reverting it and dealing with the places that were relying on it (live, I suspect) another way.

Details: The underlying cause of the behavior is that jQuery.fn.init sets the context to the DOM element (line 110 of v1.4.4; currently ​line 91 of core.js), and then add uses this.context (line 4,459 of jQuery.js v1.4.4; currently ​line 137 of traversing.js). It's probably used elsewhere as well.

[ajpiano]

I was initially inclined to think that this was related to ​7533, so I dropped the latest Git version into a jsbin as well, but the ​behaviour persists. I do agree that this is a bug - add(selector,context) was added in 1.4, but obviously the existing behaviour should not be broken.

Thanks for the report!

[T.J. Crowder]

Thanks Adam!

[dmethvin]

Agreed, it seems like a bug. The fix seems pretty easy, remove the || this.context inside the method and let it default to document.

[dmethvin]

I'll grab this one. Thanks for the report!

[T.J. Crowder]

@Dave: No worries, and thanks.

Yeah, the fix for add is easy, but I wonder if .context is being used elsewhere where it shouldn't be as well. I mean, obviously $('#first') and $(document.getElementById('first')) can't mean exactly the same thing, not if live is going to work with its current API. But a bit of an audit may be necessary to check that .context isn't being used elsewhere when it shouldn't be. I don't know the code well enough to do that. I see .context used in closest and a couple of other places, for instance, but I don't know whether that use is correct or incorrect...

[T.J. Crowder]

Amazingly, probably less than an hour after I figured out what was going on and filed this report, I ​ran into someone on StackOverflow running into it. Wow.

[dmethvin]

Fix: ​https://github.com/jquery/jquery/pull/153

[ajpiano]

#7904 is a duplicate of this ticket.

[john]

Landed.