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Show Value Instead of text in select option

I have country codes dropdown list, in select option text there countryname and ISD Code together, but I want only show ISD Code after Selection.
<select class="form-control-input" name="country_isd_code" id="country_isd_code">
<option value="">Country Code</option>
<option value="+244">Angola (+244)</option>
<option value="+1">Anguilla (+1)</option>
</select>
I have searched for some other forums but I am not able to get how to do this. like if we select Anguilla, then it should show +1 there and if it is selected Angola, it should show +244

A solution with only a <select> element.
How it works:
Initializes a hidden <option> that will be used for showing the selected option's value.
When an option is selected:
affects to the hidden option's value attribute and text content the value attribute of the just selected option if this value is not empty. Then shows that hidden option.
empties value and text content of that option, then hides it, if the chosen value is empty (Country code option here).
document.addEventListener('DOMContentLoaded', () => {
const select = document.querySelector('select');
select.addEventListener('change', () => {
const value = select.value,
showValueOption = select.querySelector('.show-value');
if (value === '') {
showValueOption.style.display = 'none';
showValueOption.value = '';
return;
}
showValueOption.style.display = '';
showValueOption.innerText = value;
showValueOption.value = value;
select.selectedIndex = 0;
});
});
<select class="form-control-input" name="country_isd_code" id="country_isd_code">
<option class="show-value" value="" style="display:none;"></option>
<option value="" selected>Country Code</option>
<option value="+244">Angola (+244)</option>
<option value="+1">Anguilla (+1)</option>
</select>

You can handle change event of select tag like this.
I updated code for display selected value as selected text.
$("#country_isd_code").change(function(){
$("#codeselect").val($(this).val());
})
$("#country_isd_code").change(function(){
$("#codeselect").val($(this).val());
//$("#country_isd_code option:selected").text($(this).val());
$("#selecteditem").val($(this).val())
$("#selecteditem").text($(this).val())
$("#selecteditem").prop('selected', true);
$("#selecteditem").show();
})
#selecteditem{
display:none;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<select class="form-control-input" name="country_isd_code" id="country_isd_code">
<option id="selecteditem" value=""></option>
<option value="">Country Code</option>
<option value="+244">Angola (+244)</option>
<option value="+1">Anguilla (+1)</option>
</select>
<input type="text" id="codeselect" />

Generate text on a webpage which depends on dropdown box options

I have created a dropdown box with 3 options on my website.
I want to change some text on the webpage, depending on the drop-down item selected. For example:
If dropdown option 1 is chosen, I would like some text on the page that says "You have chosen option 1".
I don't know any JavaScript and so I haven't tried anything apart from trying to search for a similar solution.
<select name="os0" style="background: #315d80; color: #fff; border-color: white; border: 0px;">
<option value="op1">Option 1</option>
<option value="op2">Option 2</option>
<option value="op3">Option 3</option>
</select>

Full sultion with jQuery (I started typing it before you added the HTML to your question so the options are italian car brands).
https://jsfiddle.net/mL2c2xb6/
HTML:
<select id="carSelect">
<option></option>
<option value="Audi">Audi</option>
<option value="Ferrari">Ferrari</option>
<option value="Fiat">Fiat</option>
</select>
<p id="choiceDisplay">
Selection Display
</p>
Javascript:
$('select').change(function(){ // Listen to changes to <select> element
const options = $("option"); // Get all the <option> elements.
let selectedOption = ""; // Var to store selected option value.
options.each(function(index){ // Iterate through option elements.
let option = options[index];
if($(option).prop("selected")) { // Find the seletced one.
selectedOption = $(option).val(); // Get the value and store it
};
});
$("#choiceDisplay")
.empty()
.append(selectedOption); // Display the result.
});

var e = document.querySelector('[name="os0"]');
var strUser = "";
e.addEventListener("change", function(){
strUser = e.options[e.selectedIndex].innerHTML;
alert("You have chosen " + strUser);
});

Select the first option by value when there are duplicate values

I have a select list that contains duplicate values. I don't have the power to change this.
<select id="dropdown">
<option value="1">Item 1</option>
<option value="1">Item 1 again under a different name</option>
<option value="2">Item 2</option>
</select>
In a different place, I have a link element that, when clicked, calls $("#dropdown").val(1). This "Item 1 again under a different name" to be selected and displayed. I want "Item 1" to be selected instead. How can I accomplish this?

To do that you can select the option element by the value attribute and use the :first selector. Try this:
var value = 1;
$('#dropdown option[value="' + value + '"]:first').prop('selected', true);
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<select id="dropdown">
<option>Please select...</option>
<option value="1">Item 1</option>
<option value="1">Item 1 again under a different name</option>
<option value="2">Item 2</option>
</select>

Try This :
$("#dropdown option[value=1]:first").prop({selected:true});

I guess there will be only one duplicate value in dropdown. First you need to find that value than select first option of same value.
var arrDuplicate = [];
var duplicateVal = 0;
$("#dropdown option").each(function () {
if (arrDuplicate.indexOf($(this).val()) > -1)
duplicateVal = $(this).val();
arrDuplicate.push($(this).val());
});
$("#dropdown option[value=" + duplicateVal + "]:first").attr("selected", "selected");

Can not set value dynamically in drop down using javascript/jquery

I have one issue.I want to set value in drop down by onchange event using javascript/Jquery. I am explaining my code below.
<h1>drop down</h1>
<p>
<select id="leaveCode" name="leaveCode" onChange="SetValue();">
<option value="10">Annual Leave</option>
<option value="11">Medical Leave</option>
<option value="14">Long Service</option>
<option value="17">Leave Without Pay</option>
</select>
</p>
<p>
<select id="leaveCode1" name="leaveCode">
<option value="">select value</option>
<option value="21">xyz</option>
<option value="21">abc</option>
</select>
</p>
function SetValue(){
var value=document.getElementById('leaveCode').value;
var name=document.getElementById('leaveCode').name;
document.getElementById('leaveCode1').value=value;
document.getElementById('leaveCode1').name=name;
}
Here my requirement is when user will select any value from 1st drop down list it will display also in second dropdown list in disable mode.Please help me.

You can use following statement that will append and make select in leavecode1 dropdown.Use following statements in your js file
jQuery('#leaveCode').change(function(){
//get selected text from #leaveCode list
text = jQuery("#leaveCode option:selected").text();
//get selected value for selected text
value = jQuery("#leaveCode option:selected").val();
//append new in #leavecode1 and make this option as selected option.
jQuery("#leaveCode1").append("<option value=" + value + " selected>" + text + "</option>");
});
You can check this using following link(updated)-http://jsfiddle.net/aecrcvt2/2/

The best way is to Append the new value in your dropdownlist on change the leaveCode dropdownlist
$("#leaveCode").change(function(){
$("#leaveCode1").append("<option value=" + $("#leaveCode").val() + ">" + $("#leaveCode option:selected").text() + "</option>")
});

I believe this is the answer you need:
$("#leaveCode").change(function(){
var selectedOptionValue = $(this).find(":selected").val();
var selectedOptionText = $(this).find(":selected").text();
var newOption = '<option value='+selectedOptionValue+' disabled>'+selectedOptionText+'</option>';
$("#leaveCode1 option").last().after(newOption);
});
JSFIDDLE

function SetValue(obj) {
var $this = $(obj);
var val = $this.val();
var txt = $('option:selected', $this).text();
$('#leaveCode1').append($('<option></option>').val(val).html(txt));
$('#leaveCode1').val(val).prop('disabled', 'disabled');
}
SetValue('#leaveCode');
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<h1>drop down</h1>
<p>
<select id="leaveCode" name="leaveCode" onChange="SetValue(this);">
<option value="10">Annual Leave</option>
<option value="11">Medical Leave</option>
<option value="14">Long Service</option>
<option value="17">Leave Without Pay</option>
</select>
</p>
<p>
<select id="leaveCode1" name="leaveCode">
<option value="">select value</option>
<option value="21">xyz</option>
<option value="21">abc</option>
</select>
</p>

How to get the last value selected from dropdown and disable it using jquery

I have two drop-down's, i want to disable the last value selected from the drop down and enable the previous value in other drop-down.
Let's say i have two drop-down's as below :
<select name="g1" id="box_g1">
<option value="Select">Select</option>
<option value="one">one</option>
<option value="two">two</option>
<option value="three">three</option>
</select>
<select name="g2" id="box_g2">
<option value="Select">Select</option>
<option value="one">one</option>
<option value="two">two</option>
<option value="three">three</option>
</select>
If i select option : 'one' from box_g1 and then 'two' from box_g1 again (i.e. from the same box). then only the last value 'two' should be disabled in the next drop-down. Check the below fiddle :
JS Fiddle
Any help is appreciated.

Can try something like below
<script type="text/javascript">
var g1 = [],
g2 = [];
$('#box_g1 option').each(function(){g1.push($(this).val());});
$('#box_g2 option').each(function(){g2.push($(this).val());});
$('#box_g1').on('change', function(){
var options = '',
selected = $('#box_g1').val();
for(var i = 0; i < g2.length; i++){
if(selected != g1[i] && selected != 'Select'){
options += ('<option value="'+g2[i]+'">'+g2[i]+'</option>');
}
}
$('#box_g2').empty().html(options);
});
</script>

You could store the value of the last item selected from the first box, and then iterate through the 2nd box and compare the values to the stored value, disabling all those that match.