What is the reason for having two elements with identical IDs on a single page?

While exploring a web page, I stumbled upon the following code snippet (pay attention to the id of the span element):

<div id="UpdatePanel3">     
    <span id="Timer3" style="visibility:hidden;display:none;"></span>
    <select name="DropDownList1" id="DropDownList1" class="form-control" 
        size="2" onchange="navFromList(this.value);">
        <option value="12720">Blawblawblaw</option>
    </select>                                           
</div>

Further down on the same page, within a CDATA section, I found this piece of code (take note of the uniqueID attribute):

Sys.Application.add_init(function() {
    $create(Sys.UI._Timer, {"enabled":true,"interval":15000,"uniqueID":"Timer3"}, null, null, $get("Timer3"));
});

I was puzzled by how these elements can share the same Id. It seems to be related to AJAX or partial page refreshes, but I have yet to grasp the concept fully.

If anyone could shed some light on this for me, that would be greatly appreciated!

javascriptasp.netajax

Answer №1

Using the $create method initiates the creation of a new instance of Sys.UI._Timer, which is then linked to a specific element within the DOM hierarchy. It's important to note that the timer itself does not exist as a tangible DOM element, but rather as a JavaScript entity. The value assigned to its uniqueID property mirrors the id of the corresponding DOM object for ease of reference.

Multiple JavaScript objects can be generated throughout the lifespan of the page, each with their own "id," "ID," or "uniqueID" properties set to identical values. These properties are distinct from your DOM structure and do not cause any conflicts or overlaps.

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