jQuery .data() does not work, but .attr() does

I ran into a similar "bug" a few days ago when working with .data() and .attr('data-name') for HTML5 data attributes.

The behavior you're describing is not a bug, but is by design.

The .data() call is special - not only does it retrieve HTML5 data attributes it also attempts to evaluate/parse the attributes. So with an attribute like data-myjson='{"hello":"world"}' when retrieved via .data() will return an Object while retrieval via .attr() will return a string. See jsfiddle example.

Since .data() does extra processing jQuery stores the results of attribute evaluation in $.cache - after all, once a data attribute has been evaluated it would be wasteful to re-evaluate on every .data() call - especially since data variables can contain complex JSON strings.

I said all that to say the following: After retrieving an attribute via .data() any changes made by .attr('data-myvar', '') will not be seen by subsequent .data() calls. Test this out on jsfiddle.

To avoid this problem don't intermix .data and .attr() calls. Use one or the other.

This is the result of a misunderstanding: data is NOT an accessor for data-* attributes. It's both more and less than that.

data is an accessor for jQuery's data cache on the element. That cache is initialized from data-* attributes if there are any present, but data never writes to the attributes, nor does changing the attribute change the data cache after initialization:

const div = $("[data-example]");
console.log('div.data("example"):', div.data("example"));
console.log('div.attr("data-example"):', div.attr("data-example"));
console.log('Using div.data("example", "updated")');
div.data("example", "updated");
console.log('div.data("example"):', div.data("example"));
console.log('div.attr("data-example"):', div.attr("data-example"));

<div data-example="initial value"></div>

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

data also massages what it finds in various ways, guessing at data types, making data("answer") on an element with data-answer="42" a number, not a string, or even parsing things as JSON if they look like JSON:

console.log(typeof $("[data-answer]").data("answer"));
console.log(typeof $("[data-json]").data("json"));
console.log(typeof $("[data-str]").data("str"));

<div data-answer="42"></div>
<div data-json='{"answer":42}'></div>
<div data-str="example"></div>

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

If you want to use the attributes (both reading and setting them), use attr, not data. attr is an accessor for attributes.

.attr("data-itemname", "someValue") modifies the DOM.

.data("itemname", "someValue") modifies the jQuery cache.

To get this working in following Javascript and in addition in CSS you have to set both.

theaterA.find(".someLink").attr("data-itemname", "someValue");
theaterA.find(".someLink").data("itemname", "someValue");

That's because the attribute's name is data-itemname. You cannot use - in the shorthand obj.attribute notation (obj.data-itemname would be intepreted as "obj.data minus itemname").

Why don't you just use .data() everywhere?

You can also declare default values inline on the HTML, which is fine too.

<span data-code="pony">text</span>

and

$("span").data("code") == "pony" // true

if you want to change it you just do

$("span").data("code", "not-a-pony");

and to remove it altogether you could invoke

$("span").removeData("code");

you should really try and avoid using .attr("data-*"), I don't know why you'd want to do so anyways.