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jQuery caches elements selected?

So I'm using this code to:
$('.something').on('click', function () {
console.log($(this).data('id'));
}
And for some reason, if I modify the data-id using the inspector, jQuery still sees the id that was there in the beginning. However, I tried the same thing using JS and it does see the changes. This makes me wondering if jQuery caches in some way the elements selected and uses them instead of the actual DOM.
Can someone please explain what happens and how jQuery does the event binding in the background?
Later edit: I want to specify that I'm talking about the "data-" attribute that I put in the HTML, not about the '.data()' provided by jQuery. Not sure if it's the same thing.

jQuery caches elements selected?
No. But the data managed by data is stored in an object cache maintained by jQuery, keyed by a unique identifier jQuery adds to the element (so it can look up the data). data is only initialized from data-* attributes, it is not an accessor for them. It's both more and less than that.
If you're interested, you can see that as an "expando" property on the element instance, it'll start with "jquery" and have a long number attached to it (currently; it's undocumented — for good reason — so this may change):
var foo = $("#foo");
console.log(foo.data("info")); // hi there
console.log("Expando name: " + Object.getOwnPropertyNames(foo[0]).find(name => name.startsWith("jQuery")));
<div id="foo" data-info="hi there"></div>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

Where does jquery put .val() on a DIV?

jQuery (all versions tested up through 2.1.0) allows me to call .val("some value") on a DIV object to set a value on the DIV. It is not displayed and doesn't show up as an HTML5 data property in Chrome Developer Tools. And yet I can fetch the result later with a call to .val().
For example (from http://jsfiddle.net/X2nr6/ ):
HTML:
<div id="mydiv" style="display: none;">Some text</div>
<div id="debug"></div>
Javascript:
$('#mydiv').val('A value attached .');
$('#debug').text( $('#mydiv').val() );
Displayed result:
A value attached.
Where is the value stored? Not knowing where it is stored makes me worry that I am relying on a hack.

jQuery is just assigning to a value property on the div object (the HTMLDivElement instance for that div), even though it doesn't normally have one. Creating new properties on elements is allowed in every browser I've ever seen, so it works. I wouldn't use val with divs on a regular basis, though.
Here's a non-jQuery example:
var div = document.createElement('div');
console.log('value' in div); // false, divs don't normally have a value property
div.value = 42;
console.log('value' in div); // true, we've created a property on the element
console.log(div.value); // 42
Or the same sort of thing using jQuery:
var $div = $("<div>");
display(typeof $div.prop('value'));
$div.val(42);
display(typeof $div.prop('value'));
display($div.prop('value'));
This business of creating new, custom, non-standard properties on elements is called creating "expando" properties. They can be very handy. (jQuery uses them internally, for instance, to manage the data cache and a few other things — if you look closely at a DOM element you've set data on using data, you'll see a property with a name like jQuery1110028597884019836783; that's the key jQuery uses to find the element's data in jQuery's internal data cache. jQuery doesn't store the data in an expando on the element, because of IE garbage collection issues; it stores the key there, and the data in a JavaScript object.)

It stores it on a value property on the DOM object. You can see if by running your code and then inspecting the element in a DOM inspector. In Chrome, the value property will be listed under div#mydiv in the properties tab.
HTMLDivElement objects don't officially support such a property, so you are relying on a hack.
Use data() to store arbitrary data on an element.
$('#mydiv').data("myCustomValue", 'A value attached .');

Although the above answers are accurate, I'd like to complete something out.
jQuery is designed around the concept of wrapping all HTML elements in the jQuery object. That jQuery object happens to be an array that can hold more than one element.
jQuery also goes out of its way to hide this fact from you so that the average jQuery developer never has to worry about exactly what he has -- simply call the right method and the magic happens.
(You see this if you do a $(".someClassYouHaveLotsOf").hide() or $(".someClassYouHaveNoneOf").hide()`.)
jQuery's val() method is just a wrapper for accessing an HTML input element's value property. Since jQuery doesn't throw errors unless there is really no way what-so-ever, it silently helps you by accessing the value property on whatever HTML element it happens to have. div span or whatever.
In most browsers, this works -- mostly enough.
If you are really interested in setting values on HTML elements for use later, the data() method is far better suited. Straight HTML would use <element>.setAttribute("data-key", "value");
And that is about the only time you'll see me using the HTML attributes over properties, BTW.

Issue while accessing element by dojo.byId dojo 1.8

I am trying to access a div element text by using dojo.byId but it is returning the value which is set at first time the value is selected.It is somehow binding the value initially selected to the id of div and hence returning the same value even after I change the value to some other value.
var startDateLabel = dojo.byId("startDateLabel");
<label class="secondaryColor bold75Font floatRight" id="startDateLabel">${startDate} </label>
I tried to use registry.ById but since it is in a widget that is created more than once , it gives "id already registered error".For removing that , I also used destroyRecursive method but that also doesn't work.
Earlier, I used the id of container in which the widget is loaded and traversed to the children hierarchy to get the label value and it worked fine. but the child traversal code made it a bit messy.Something like
var startDateCont = registry.byId("startDateContainer");
var startDateLabel = startDateCont.domNode.children[1].children[1].children[1].innerHTML;
Is there any other way in dojo to achive this????

If you're using this inside a widget you should not even use IDs (or at least use generated IDs). The best way to get a reference to a DOM node while using a widget is by using attach points. An example, consider the following HTML:
<label class="secondaryColor bold75Font floatRight" data-dojo-attach-point="startDateLabel">${startDate} </label>
As you can see I introduced an attribtue called data-dojo-attach-point which allows me to give a name to the label (similar to an ID).
Then inside the widget you can now easily get a reference to that DOM node by using:
this.startDateLabel;
Just some extra information, you can also define events in a similar way by using data-dojo-attach-event. Make sure to read this part of the Writing your own widget article.

browsing through nodes

this is what an html structure of the webpage looks like:
<body>
<form>
<input type='file'/>
</form>
<div id='list'>
<div>value here<input id='delete' type='button'/></div>
</div>
</body>
i have found javascript code that triggers on 'delete' button click and removes input 'file' element. it uses this piece of code where element is input 'file' mentioned above:
deleteButton.onclick=function(){this.parentNode.element.parentNode.removeChild(
this.parentNode.element );}
i am trying to understand logic(rules) behind 'this.parentNode.element' ? why not accessing element directly 'element.parentNode.remove...'
many thanks

i am trying to understand logic(rules) behind 'this.parentNode.element' ?
There's no element property on the Node, Element, HTMLElement, or HTMLDivElement interfaces. So my guess would be that elsewhere in that code, you'll find something that's explicitly adding that property to the element instance of the div containing the button. You can do that, add arbitrary properties to element instances. These are frequently called "expando" properties and should be done very, very, very carefully.

Not the answer to the question, just opinion. It's better avoid constructions like
this.parentNode.element.parentNode
Because in case when you change your DOM structure, you will need rewrite you JS. So I think it's better to give id attributes to tags, and use next construction to get DOM element:
document.getElementById('element_id')
or if you will use some js framework (like jQuery) you can use even easier construction to get DOM element
$("#ement_id")

Ok, "removeChild" is a strange method, and quite probably, ill-conceived. It should look like:
<div>value here<input id='deleteMe' type='button'/></div>
var node = document.getElementById('deleteMe');
node.remove(); // <--- does not exist, but sure would be nice!!!
No, instead we have to do these shenanigans:
var node = document.getElementById('deleteMe');
node.parentNode.removeChild(node); // verbose! Convoluted!
We have to get the node's parent, call the method, then refer to the node again. This doesn't look like any other DOM methods as far as I recall. The good news is you can make it happen all in one line, chained, like a jQuery method.
You are best served to start over or copy somebody else's code. The use of "this" means it was within an object (or class), referring to other methods or properties within that object. You should stick to non-object variables and functions for now.
Hope that helps.

How can I apply prototype's toggle to content loaded via JSON, IE problem only

I use PHP and javascript via prototype.
i have a threaded comments page that on open
by default via javascript call to a PHP file data returned via JSON, only parent comments are retrieved in the db. (in short only parent comments are fetched from db and displayed)
then the parents are loaded and formatted to be shown on the page with a link to get and display its child comments, link example:
<span id="moreparent8351" onclick="insertChildDiv('parent8351')">1 Replies and more</span>
the span link above calls the javascript function "insertChildDiv()" that basically gets comments whose parent_id=parent8351, also using a PHP file that returns data via JSON to the javascript that dynamically inserts this child comments nested under its parent comment.
then the span link above using prototype transforms into:
<span id="moreparent8351" onclick="$('childparent8351').toggle()">1 Replies and more</span>
now here is the problem, this inserted content inside this div with id=childparent8351 wont respond to the hide/show toggle ONLY in IE v6,v7 and v8. other browsers work fine.
it looks like IE cannot apply the hide/show toggle to a dynamically inserted content.
I tried hardcoding the inserted content that i see in fogbugz to the page and testing it again on IE, guess what? toggle works!
I dont want to fetch all the comments both parent and child and then hide the child comments, that is a waste of resources on something that we are not sure is important or to be read by the users.
Is there a workaround? if there is none, then i hope this post will help others on their design stage for something similar.

Element.toggle() should work fine with dynamically generated content. Your problem most likely lies within the method you use to generate this content.
You stated:
then the span link above using
prototype transforms into:
<span id="moreparent8351" onclick="$('childparent8351').toggle()">1 Replies and more</span>
How exactly are you changing the onclick attribute of the span element?
First of all, I definately would not recommend changing an onclick event on-the-fly. It is probably better to use one function that checks the current state of what you are trying to do and executes the appropriate code.
Be sure not to "transform" your span element, including the onclick attribute using innerHTML. Chances are the DOM is still trying to reference a function that you have just removed. If updating your onclick attribute at all, do it like this:
var element = $('moreparent8351');
// this automatically removes previous onclick handlers set in this manner
element.onclick = function() { // do stuff };
...or, when you want to it the Prototype way:
var element = $('moreparent8351');
// OPTIONAL: remove previous handler, if set
element.stopObserving('click', my_cool_function());
// attach new handler
element.observe('click', function() { // do stuff });