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JQuery to get the last changed value from #Html.DropDownListFor asp.net mvc multiselect dropdownlistbox

How do I use Jquery to find the last checked/unchecked item and so that I can add or remove them from other two listboxs?
I am creating a dropdown listbox(excludedPeople) with multiselect checkbox with two other listboxs(PrimaryPerson,secondaryPerson) in same form. All three list box are having same set of data during form load. If any item in excludedPeople is selected(checked), I need to remove that item from PrimaryPerson and secondaryPerson and vise-versa.
ASP.Net MVC multiselect Dropdown Listbox code:
#Html.ListBoxFor(m => m.ExcludedPeople, Model.AllPeopleListViewModel,
new { #class = "chkDrpDnExPeople" , #multiple = "multiple"})
jQuery code:
$(".chkDrpDnExPln").change(function ()
{
console.log("Trigger" + $(this).val()); //this code gets the list of all items selected. What I need is to log only last selected/unselected item's val & text into the console.
});
Any help is appreciated. Ask questions if any.

Well, after waiting for 2 days I made a solution myself and posting it here so that others can make use of it.
I made this code for multiselect dropdown listbox with checkboxes in each list item. I expect this to work on similar controls like checked listbox but haven't tested it.
I followed register control and get notified by event so the usage can be made seamless without getting into details.
Usage:
1) include the "JQuery based Library" part into your project as shared or same js script file.
2) Use the below approach to consume the functionality. The event should get you the changed values when the control selection is changed.
RegisterSelectedItemChangeEvent("chkDrpDnctrl#1");
RegisterSelectedItemChangeEvent("chkDrpDnctrl#2");
RegisterSelectedItemChangeEvent("chkDrpDnctrl#3");
$(".chkDrpDnctrl").on("OnSelectionChange", function (e,eventData)
{
var evntArgs = {
IsDeleted: false,
IsAdded: false,
AddedValues: [], //null if no change/None. Else changed value.
DeletedValues: [] //null if no change/None. Else changed value.
};
var source = e;
evntArgs = eventData;
var elementnm = $(this).attr("id");
if (evntArgs !== "undefined" && elementnm != "")
{
if (evntArgs.IsAdded == true)
{
//if excluded checked then remove.
for (var i = 0; i < evntArgs.AddedValues.length; i++)
{
PerformAction (control#, evntArgs.AddedValues[i]);
}
}
if (evntArgs.IsDeleted == true)
{
//if excluded checked then remove.
for (var i = 0; i < evntArgs.DeletedValues.length; i++)
{
PerformAction (control#, evntArgs.AddedValues[i]);
}
}
}
});
JQuery based Library:
function RegisterSelectedItemChangeEvent(selector) {
var dropdownElementRef = selector;
//Intializes the first time data and stores the values back to control. So if any of the checkboxes in dropdown is selected then it will be processe and added to control.
$(dropdownElementRef).data('lastsel', $(dropdownElementRef).val());
var beforeval = $(dropdownElementRef).data('lastsel');
var afterval = $(dropdownElementRef).val();
//storing the last value for next time change.
$(dropdownElementRef).data('lastsel', afterval);
//get changes details
var delta = GetWhatChanged(beforeval, afterval);
//stores the change details back into same object so that it can be used from anywhere regarless of who is calling it.
$(dropdownElementRef).data('SelectionChangeEventArgs', delta);
//prepares the event so that the same operation can be done everytime the object is changed.
$(dropdownElementRef).change(function () {
var beforeval = $(dropdownElementRef).data('lastsel');
var afterval = $(dropdownElementRef).val();
//storing the last value for next time change.
$(dropdownElementRef).data('lastsel', afterval);
//get changes details
var delta = GetWhatChanged(beforeval, afterval);
//stores the change details into same object so that it can be used from anywhere regarless of who is calling it.
$(dropdownElementRef).data('OnSelectionChangeEventArgs', delta);
//fires the event
$(dropdownElementRef).trigger('OnSelectionChange', [delta]);
//$.event.trigger('OnSelectionChange', [delta]);
});
var initdummy = [];
var firstval = GetWhatChanged(initdummy, afterval);
//fires the event to enable or disable the control on load itself based on current selection
$(dropdownElementRef).trigger('OnSelectionChange', [firstval]);
}
//assume this will never be called with both added and removed at same time.
//console.log(GetWhatChanged("39,96,121,107", "39,96,106,107,109")); //This will not work correctly since there are values added and removed at same time.
function GetWhatChanged(lastVals, currentVals)
{
if (typeof lastVals === 'undefined')
lastVals = '' //for the first time the last val will be empty in that case make both same.
if (typeof currentVals === 'undefined')
currentVals = ''
var ret = {
IsDeleted: false,
IsAdded: false,
AddedValues: [], //null if no change/None. Else changed value.
DeletedValues: [] //null if no change/None. Else changed value.
};
var addedvals;
var delvals;
var lastValsArr, currentValsArr;
if (Array.isArray(lastVals))
lastValsArr = lastVals;
else
lastValsArr = lastVals.split(",");
if (Array.isArray(currentVals))
currentValsArr = currentVals;
else
currentValsArr = currentVals.split(",");
delvals = $(lastValsArr).not(currentValsArr).get();
if (delvals.length > 0)
{
//console.log("Deleted :" + delvals[0]);
for (var i = 0; i < delvals.length; i++)
{
ret.DeletedValues.push(delvals[i]);
}
ret.IsDeleted = true;
}
addedvals = $(currentValsArr).not(lastValsArr).get();
if (addedvals.length > 0)
{
//console.log("Added:" + addedvals[0]);
for (var i = 0; i < addedvals.length; i++)
{
ret.AddedValues.push(addedvals[i]);
}
ret.IsAdded = true;
}
return ret;
};

Set value/text to dropdown list

I am using the following jquery postcode lookup and I am trying to push values and hard code the process in javascript.
I am using the following to give the postcode textbox a valid postcode, and then forcing the button click to find the addresses.
document.getElementById("idpc_input").value = "LL17 0PN";
document.getElementById('idpc_button').click();
This so far works, after this a dropdownlist appears with the id of 'idpc_dropdown', I am trying to (in javascript or jquery) select an option
Here is what I have done but it does not work
var select = document.getElementById("idpc_dropdown");
document.getElementById("idpc_dropdown").text = '2 Elwy Cottages Heol Esgob';
And also:
var desiredValue = "2 Elwy Cottages Heol Esgob"
var el = document.getElementById("idpc_dropdown");
for(var i=0; i<el.options.length; i++) {
if ( el.options[i].text == desiredValue ) {
el.selectedIndex = i;
break;
}
}
UPDATE:
Let me explain the process and order, 1- Type in postcode and press the button to find my address 2- a dropdownlist then renders and appears .. I Think this is why it is not working for my desired dropdownlist as its not loaded when the page is loaded, it is when the button has been pressed

Provided that i is the same as the index of the item you wish to select, I'd set the dropdown's value attribute to that index:
let el = document.getElementById("idpc_dropdown");
for(let i = 1; i <= el.options.length; i++) {
if ( el.options[i].text == desiredValue ) {
el.value = i; // here
break;
}
}

Loop through checkboxes that are not hidden

On a webpage I give the user the option of hiding table elements (which contain checkboxes) like this:
mytable.style.display = 'none'; //the table and the enclosed textbox is hidden
I am now trying to find all of the tables that are not hidden like this:
var frm = document.forms[0];
var arrayDisposals;
var intCount;
var arrayDisposals = new Array();
for (i = 0; i < frm.elements.length; i++) {
if (frm.elements[i].type == "checkbox" && frm.elements[i].name.substr(0, 3) == "Del") {
if ('none' != frm.elements[i].style.display) {
{
arrayDisposals.push(frm.elements[i].id + '|' + frm.elements[i].checked)
}
}
}
The problem is that the second IF statement does not work i.e. all elements are added to the array. How do I only add checkboxes that are not hidden?

If you were looking for a jQuery solution this should suffice. Use .map()
var arrayDisposals = $('input[type="checkbox"][name*="Del"]:visible').map(function(){
return this.id+ "|" + this.checked
}).get();
Use attribute selector to find checkboxes and where the name contains "Del" and :visible to check if it is not hidden.
DEMO

How can I select all elements with the same class name?

I have a Boolean variable. It is stored in a hidden input field. Basically, if the user is signed in, it is false, if not, it is true.
There are download buttons which will link to a file download. My aim is to make it so that, if they aren't signed in, the button will not show, and the link will not work (it would be nice to have an alert saying they need to sign in or something, but that would probably be more effort than it's worth).
I have a function that performs onload of body:
function hide_download_btns(){
if (document.getElementById('download_btn_var_input').value == "true") {
document.getElementsByClassName('project_download_btn').item(0).hidden = true
}
}
My problem is where it asks for the nth term .item(0). This is where it selects the div on which to perform the function, however, I want the function to affect all divs with the class name 'project_download_btn'.
I'm not a fan of jQuery, so it would be great to avoid that if possible.

You can simply loop through the elements instead of just taking the 0th.
var buttons = document.getElementsByClassName('project_download_btn');
for(var i=0; i< buttons.length; i++){
buttons[i].hidden = true;
}

if (document.getElementById('download_btn_var_input').value == "true") {
var el = document.getElementsByClassName('project_download_btn');
for (var i = 0; i < el.length; i++) {
el[i].hidden = true;
}
}

document.getElementsByClassName returns array so what you are interested is :
document.getElementsByClassName('project_download_btn')[0]

Loop through each div that contains your download button and set hidden to true:
if (document.getElementById('download_btn_var_input').value == "true") {
var button_divs_array = document.getElementsByClassName('project_download_btn');
for (var i = 0; i < button_divs_array.length; i++) {
button_divs_array[i].hidden = true;
}
}

How Can I Check the checkbox that belongs a GridVIew?

I want to do a javascript function that check the unchecked checkbox. My function nowadays, check all the unchecked checkbox, and I need that just check a specific GridView unchecked checkbox
function checar() {
var el = document.getElementsByTagName("input");
for (var i = 0; i < el.length; i++) {
if (el[i].type == "checkbox") {
el[i].checked = true;
}
}
}

You want to first limit the scope of your search for elements. One way to do so would be to use getElementById
function checar() {
var grd = document.getElementById("<%=grd.ClientID%>"); // <-- Add this line
var el = grd.getElementsByTagName("input"); // <-- change the scope of this to grd
//rest of your code here.
}
Sample using divs, but you'll get the idea I think: http://jsfiddle.net/8LRkk/
Edited to include setting the specific Grid ID.

To get at all of the checkboxes of a particular gridview, you need to grab checkboxes whose ClientID contain the portion of the gridview's ClientID as well since all controls have an id which is "stacked".
Your function should work as a base, it just needs to have an added check in it:
function checar() {
var el = document.getElementsByTagName("input");
// Get the client id of the gridview from ASP here
var gvID = '<%= this.myGridview.ClientID %>';
for (var i = 0; i < el.length; i++) {
// Call the id of the checkbox and check to see if it
// contains the client id of the gridview at the same time
if (el[i].type == "checkbox" && el.id.indexOf(gvID) != -1) {
el[i].checked = true;
}
}
}