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

Forgive me for not being more specific on this. I have such a strange bug. After the doc loads, I loop some elements that originally have data-itemname="", and I set those values using .attr("data-itemname", "someValue").

Issue: When I later loop thru those elements, if I do elem.data().itemname, I get "", but if I do elem.attr("data-itemname"), I get "someValue". It's like jQuery's .data() getter only gets elements that are set initially to contain some value, but if they are originally empty, and later set, then .data() doesn't get the value later on.

I've been trying to recreate this bug but have not been able to.

Edit

I have recreated the bug! http://jsbin.com/ihuhep/edit#javascript,html,live

Also, snippets from above link...

JS:

var theaters = [
    { name: "theater A", theaterId: 5 },
    { name: "theater B", theaterId: 17 }
];

$("#theaters").html(
    $("#theaterTmpl").render(theaters)
);

// DOES NOT WORK - .data("name", "val") does NOT set the val
var theaterA = $("[data-theaterid='5']");
theaterA.find(".someLink").data("tofilllater", "theater5link"); // this does NOT set data-tofilllater
$(".someLink[data-tofilllater='theater5link']").html("changed link text"); // never gets changed

// WORKS - .attr("data-name", "val") DOES set val
var theaterB = $("[data-theaterid='17']");
theaterB.find(".someLink").attr("data-tofilllater", "theater17link"); // this does set data-tofilllater
$(".someLink[data-tofilllater='theater17link']").html("changed link text");

HTML:

<body>
    <div id="theaters"></div>
</body>

<script id="theaterTmpl" type="text/x-jquery-tmpl">
    <div class="theater" data-theaterid="{{=theaterId}}">
        <h2>{{=name}}</h2>
        <a href="#" class="someLink" data-tofilllater="">need to change this text</a>
    </div>
</script>
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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.


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