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Similar selects, how to know which one is changed?

I have a page where rows can be added which include drop downs (select) fields. each one of those fields is dynamically created with an id of "drop1, drop2, drop3" etc.
I'm wanting to achieve pulling some data from mysql with ajax, problem i'm having is knowing which drop down has changed to pull the data.
ive used this to know how many select fields there are currently, but don't know how to decide which one has changed. any help is appreciated.
var myCount = $("select[id^=drop]").length;
here is the row that gets added.
var n=($('.detail tr').length-0)+1;
var tr = '<tr>'+
'<td>'+n+'</td>'+
'<td><select id="drop'+n+'" name="prodService[]"></select></td>'+
'<td id="desc'+n+'"></td>'+
'<td>Delete</td>'+
'</tr>';

The whole idea around the select input it's like any input accepts all events. Any input takes click, change, input...etc let's say we have 3 select dropdown.
How to detect if any of them has changed and get the new value
$('.select-list').on('change', function(evt) {
console.log($(this).val()); // get the select value - new value if changed
console.log($(this).attr('id')); // just for implementation purposes and debugging
});
Going further you can implement something like: When the user selects an item from a dropdown, send a request to a server to get some data. You can do that with the value="" attribute, put keyword/key-trigger (just to differentiate between them) to send different request depending on what item has been selected.
$('.select-list').on('change', function(evt) {
console.log($(this).val()); // get the select value - new value if changed
console.log($(this).attr('id')); // just for implementation purposes and debugging
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<select class="select-list" name="items" id="itemsList1">
<option value="1">item1</option>
<option value="2">item2</option>
<option value="3">item3</option>
<option value="4">item4</option>
</select>
<select class="select-list" name="items" id="itemsList2">
<option value="1">item1</option>
<option value="2">item2</option>
<option value="3">item3</option>
<option value="4">item4</option>
</select>
<select class="select-list" name="items" id="itemsList3">
<option value="1">item1</option>
<option value="2">item2</option>
<option value="3">item3</option>
<option value="4">item4</option>
</select>

Have dropdown menus use value of previous dropdown

I have a "master dropdown menu" that contains four options (English, Spanish, Russian, Other) as the question asks their primary language.
Below this master dropdown menu contains other questions that asks similar questions and contain the same options (English, Spanish, Russian, Other).
I am wondering if it's possible to have the other dropdown menu options have the same value selected based on the 'master dropdown menu' option. So if John Smith chooses English in the master dropdown menu, the other questions will automatically have English selected in the other dropdown menus, while also allowing John to change an Answer - he may choose Spanish for question 3. I'm hoping the solution uses JavaScript or jQuery, as PHP won't be an option.
<label>What is your primary language?</label>
<select name="question1">
<option></option>
<option value="english">English</option>
<option value="spanish">Spanish</option>
<option value="russian">Russian</option>
<option value="other">Other</option>
</select>
<label>Language your spouse speaks?</label>
<select name="question2">
<option></option>
<option value="english">English</option>
<option value="spanish">Spanish</option>
<option value="russian">Russian</option>
<option value="other">Other</option>
</select>
<label>Language your children speak?</label>
<select name="question3">
<option></option>
<option value="english">English</option>
<option value="spanish">Spanish</option>
<option value="russian">Russian</option>
<option value="other">Other</option>
</select>

When selecting the value in the master dropdown menu you can set the value to the other dropdowns via JavaScript.
document.querySelector('select[name="question1"]').addEventListener('change', function(event) {
document.querySelector('select[name="question2"]').value = event.target.value;
document.querySelector('select[name="question3"]').value = event.target.value;
});
really simple solution but should work
JSFiddle

You can do this in jQuery as follows, using nextAll() to populate the other select items:
$('select').on('change', function() {
$(this).nextAll('select').val($(this).val());
});
(Note that you probably want to add a class to your selects as the above will operate on ALL select elements on the page).

Below you are adding an EventListener to to trigger when the first item is changed and assigning its value to rest of the selectors. Please update var firstDD and reminingDD valriables based on your requirement. Or better use specific IDs for each seletor so that it would be easy to understand.
$( document ).ready(function() {
var firstDD = $("select:first-child");
var reminingDD = $("select:not(:first-child)");
console.log(reminingDD);
firstDD.change(function () {
$(reminingDD).val($(firstDD).val());
});
});

What is the best practice to scale a select/dropdown

I have a long list (of an undetermined length, that can keep on growing) of values. For example:
<select>
<option value="1">1, some test</option>
<option value="2">2, some text</option>
<option value="3">3, some text</option>
<option value="4">4, some text</option>
<option ...... add 100 more options> ..
</select>
What would be the best practice/user friendly way to support such a long list? I imagine that a user might find it frustrating to scroll through a super long list.
I am trying to minimize the user's scroll time, by presenting the most recent values at the top of the drop down, but if she/he would like to go to the first element in the drop down it would require a lot of scrolling.
Thanks

Instead of using <select>, how about changing it to HTML5 <datalist>, which is like this:
<datalist id="browsers">
<option value="Internet Explorer">
<option value="Firefox">
<option value="Chrome">
<option value="Opera">
<option value="Safari">
</datalist>
By the way, you can type the first letter using the <select>, so my suggestion will be sort your data according to the first letter. Because it will help the user to go to the sub-option directly.

Maybe add a searchfield to the dropdown?
Something like this: http://jsearchdropdown.sourceforge.net/

What kind of data would you like to display ? Maybe there is another way than select to display them ? In fact, for dozens of items, select is not the best choice, unless you use by example a JQuery plugin like jsearchdropdown and in this case the user must know what he's looking for.

Dojo: How to disable a dijit.form.FilteringSelect option

I am trying to disable option items in a dijit/Form/FilteringSelect. Here is code
<select id="filtSelect" dojoType="dijit.form.FilteringSelect">
<option disabled="disabled">Select</option>
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</select>
Please help me.

You can do it this way: ( Vers. 1.9 )
<select data-dojo-type="dijit/form/" id="count" name="count">
<option disabled="disabled">Select</option>
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</select>
Here's the full fiddle for my Example above(Edited by Thomas Upton): http://jsfiddle.net/tupton/266C4/
and here the reference from dojo: http://dojotoolkit.org/reference-guide/1.9/dijit/form/FilteringSelect.html?highlight=filteringselect
In the 1.6 Vers it must look like:
<script>
dojo.require("dijit.form.FilteringSelect");
</script>
<body class="claro">
<select dojoType="dijit.form.FilteringSelect" id="fruit" name="fruit">
<option value="AP" disabled>
Apples
</option>
<option value="OR" selected>
Oranges
</option>
<option value="PE">
Pears
</option>
</select>
</body>
Here the fiddle for this Version: http://jsfiddle.net/Q4zw6/
It's important that you load the instance of dijit.form.filteringSelect to use it.
Regards, Miriam

I don't believe you can disable options with dijit/form/FilteringSelect because it is store-based and is supposed to let a user enter any text.
There is a property called displayedValue, which you can use to set whatever you want. However, with FilteringSelect, any text that isn't an option is marked as erroneous input, as seen by the following code and this jsfiddle.
<select data-dojo-type="dijit/form/FilteringSelect" data-dojo-props="displayedValue: 'Select'" id="count" name="count">
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</select>
You could use a dijit/form/ComboBox in exactly the same manner; the difference between FilteringSelect and ComboBox is that the latter allows any input. See the documentation for more information.

The issue is that FilteringSelect relies on dojo data store, as others have pointed out. So if you don't create that manually, it will happen behind the scenes and you just won't know how to reference it. But on second thought, looking at the API I notice FilteringSelect has a property named store.
So, you either need to create the data store yourself & initialize the FilteringSelect with it (http://dojotoolkit.org/reference-guide/1.9/dijit/form/FilteringSelect.html) or retrieve it by using dijit's byId and then accessing the "store" property.
Then, manipulate that data store. Maybe store a temporary copy before you remove the option if you want to restore it after. If you look up data store I'm sure removing an option is trivial. Sometimes with widgets it happens that you need to trigger a re-rendering method also to have it update after (you can usually find some method in the widget's API for this, sometimes "reset"), though I believe with stores it may watch them more intelligently.

Change drop down selected item based on selected item of another drop down list

I have two drop down lists on my ASP.NET MVC 3. When one of the drop down lists is set to "Sole Proprietor", the other needs to be set to the same.
I'm sure the JavaScript, or jQuery, is very simple for something like this, however I am having a hard time finding a good example on the web since I am populating the drop down lists manually instead of through the controller.
Can someone either help me out with the code or point me to a good resource?
<select id="ProducerType" name="nmf" style="float:left;">
<option value="Principal">Principal</option>
<option value="Producer">Producer</option>
<option value="SoleProprietor">Sole Proprietor</option>
</select>
<select id="Role" name="nmf" style="float:left;">
<option value="Agent">Agent</option>
<option value="Financial Advisor">Financial Advisor</option>
<option value="Platform">Platform</option>
<option value="Principla_Owner">Principal/Owner</option>
<option value="Registered Rep">Registered Rep</option>
<option value="Sole Proprietor">Sole Proprietor</option>
</select>

jsFiddle example : http://jsfiddle.net/9GZQ2/
I set both dropdown values to "Sole Proprietor" your code has the space missing in the ProducerType select
<script type="text/javascript" src="http://code.jquery.com/jquery-1.6.4.js"></script>
<script type="text/javascript">
$(function(){
$("#ProducerType").change(function(){
var value=$(this).val();
if(value=="Sole Proprietor") $("#Role").val(value);
});
$("#Role").change(function(){
var value=$(this).val();
if(value=="Sole Proprietor") $("#ProducerType").val(value);
});
});
</script>