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how to determine specific item to be selected in combobox with javascript

when I open the combobox via contextMenu, always the same item is selected.
How can I determine that one of the other items is selected?
For instance, there are 3 items "car1", "car2", "car3" and when I open the combobox "car1" appears in the list first. So how would I get e.g. "car2" appears in the list first?
Thanks very much!

if you mean a select element, you can change the order of items
<select id="list">
<option value="2">Car 2</option>
<option value="1">Car 1</option>
<option value="3">Car 3</option>
</select>
you could also set the default item with the selected attribute:
<select id="list">
<option value="1">Car 1</option>
<option value="2" selected>Car 2</option>
<option value="3">Car 3</option>
</select>

in JS you simply have to set the value of your select
const myForm = document.forms['my-form']
// set the select on the second cat
myForm.miaou.value = 'c2'
<form action="xxx" name="my-form">
<!-- other form elements -->
<select name="miaou">
<option value="c1">Cat 1</option>
<option value="c2">Cat 2</option>
<option value="c3">Cat 3</option>
</select>
</form>

Selectize - select 2nd option on load

On load, how to select second option using selectize plugin?
https://github.com/selectize/selectize.js
As per below example, it have to select 21-Nov-2017.
I can able to select option using value.. but, this value will be dynamic and will change daily.
Thanks
HTML:
<select>
<option value="20-Nov-2017">20-Nov-2017</option>
<option value="21-Nov-2017">21-Nov-2017</option>
<option value="22-Nov-2017">22-Nov-2017</option>
</select>

You can select the second value on the basis of index like this
$('#selectBox :nth-child(2)').prop('selected', true);
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<select id="selectBox">
<option value="0">Number 0</option>
<option value="1">Number 1</option>
<option value="2">Number 2</option>
<option value="3">Number 3</option>
<option value="4">Number 4</option>
<option value="5">Number 5</option>
<option value="6">Number 6</option>
<option value="7">Number 7</option>
</select>

In version 1.6 and higher the prop() method is recommended:
$('.selDiv option:eq(1)').prop('selected', true)
In older versions:
$('.selDiv option:eq(1)').attr('selected', 'selected')

jQuery Select box multiple option get value and match values

I have same classname multi select box with same values in body, what I want if use chose value 1 from first or any select box then that value 1 on all other select box option will disable so user can't choose the same value in other multi select box.
<select name="mySel" class="myselect" multiple="multiple">
<option value="val1">option 1</option>
<option value="val2">option 2</option>
<option value="val3">option 3</option>
<option value="val4">option 4</option>
</select>
<select name="mySel" class="myselect" multiple="multiple">
<option value="val1">option 1</option>
<option value="val2">option 2</option>
<option value="val3">option 3</option>
<option value="val4">option 4</option>
</select>
This need to achieve via javascript or jQuery,

So you're likely going to get downvotes. You haven't actually shown that you've tried anything, you have only given us the HTML and asked that we solve your problem.
That said, I like to sink my teeth into easy problems, as this is. What you're wanting to do is, in each select, listen for the change event. When that triggers, get the value of the selected option, and disable any option with that value in any other selects. Take a look at the following:
$(function() {
/********
* Function to disable the currently selected options
* on all sibling select elements.
********/
$(".myselect").on("change", function() {
// Get the list of all selected options in this select element.
var currentSelectEl = $(this);
var selectedOptions = currentSelectEl.find("option:checked");
// otherOptions is used to find non-selected, non-disabled options
// in the current select. This will allow for unselecting. Added
// this to support extended multiple selects.
var otherOptions = currentSelectEl.find("option").not(":checked").not(":disabled");
// Iterate over the otherOptions collection, and using
// each value, re-enable the unselected options that
// match in all other selects.
otherOptions.each(function() {
var myVal = $(this).val();
currentSelectEl.siblings(".myselect")
.children("option[value='" + myVal + "']")
.attr("disabled", false);
})
// iterate through and disable selected options.
selectedOptions.each(function() {
var valToDisable = $(this).val();
currentSelectEl.siblings('.myselect')
.children("option[value='" + valToDisable + "']")
.attr("disabled", true);
})
})
})
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<select name="mySel" class="myselect" multiple="multiple">
<option value="val1">option 1</option>
<option value="val2">option 2</option>
<option value="val3">option 3</option>
<option value="val4">option 4</option>
</select>
<select name="mySel2" class="myselect" multiple="multiple">
<option value="val1">option 1</option>
<option value="val2">option 2</option>
<option value="val3">option 3</option>
<option value="val4">option 4</option>
</select>
<select name="mySel3" class="myselect" multiple="multiple">
<option value="val1">option 1</option>
<option value="val2">option 2</option>
<option value="val3">option 3</option>
<option value="val4">option 4</option>
</select>
<select name="mySel4" class="myselect" multiple="multiple">
<option value="val1">option 1</option>
<option value="val2">option 2</option>
<option value="val3">option 3</option>
<option value="val4">option 4</option>
</select>
Edited the above code to support multiple selects. Actually, it wasn't so much an issue of multiple selects, as I was doing a very lazy handling of unselecting options. Now, when the user is selecting or de-selecting options in any select, only the non-disabled options in that select are used: the (":checked") to disable that option in all other selects, and the .not(":checked") to re-enable any options that the user has somehow unselected.

Try this code , it should do the job:
using Not selector and for each to iterate on other select options:
$(document).ready(function() {
$('.myselect:first').change(function() { // once you change the first select box
var s = $('.myselect:first option:selected').val(); // get changed or clicked value
others = $(':not(.myselect:first) >option'); //select other controls with option value
others.each(function() {
if (this.value == s) // for example if you click on 1 the other controls will get 1 disabled
$(this).attr('disabled', 'disabled').siblings().removeAttr('disabled');
// remove disabled from others if you click something else
});
});
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<select class="myselect" multiple="multiple" name="mySel">
<option value="val1">option 1</option>
<option value="val2">option 2</option>
<option value="val3">option 3</option>
<option value="val4">option 4</option>
</select>
<select class="myselect" multiple="multiple" name="mySel">
<option value="val1">option 1</option>
<option value="val2">option 2</option>
<option value="val3">option 3</option>
<option value="val4">option 4</option>
</select>

Jquery change multiple select buttons with a select button

I want to change multiple select buttons's values with another select button.
Practically when someone select an option in the main select button, the other select buttons must change with that value.
EDIT: I added the code, the first is the main select button and after I select somethig there, it should change in the others.
<select class="select_format">
<option value="0.49">9x13</option>
<option value="0.59">10X15</option>
<option value="0.99">13X18</option>
<option value="1.20">15X21</option>
<option value="2.60">20X30</option>
</select>
<select name="format[]" class="format_imgs">
<option value="0.49">9x13</option>
<option value="0.59">10X15</option>
<option value="0.99">13X18</option>
<option value="1.20">15X21</option>
<option value="2.60">20X30</option>
</select>
<select name="format[]" class="format_imgs">
<option value="0.49">9x13</option>
<option value="0.59">10X15</option>
<option value="0.99">13X18</option>
<option value="1.20">15X21</option>
<option value="2.60">20X30</option>
</select>
<select name="format[]" class="format_imgs">
<option value="0.49">9x13</option>
<option value="0.59">10X15</option>
<option value="0.99">13X18</option>
<option value="1.20">15X21</option>
<option value="2.60">20X30</option>
</select>
Thanks.

Since you didn't supply code here's a guess:
$(".myTriggerButton").on("click", function() {
$(".buttonToChangeValue").each(function() {
this.value = "new value";
});
});

$('.select_format').change(function(){
$('.format_imgs').val( $(this).val() );
});
https://jsfiddle.net/7hno0ob8/1/

Click on option event

how do I handle events for option elements?
<select>
<option value='option1'>Gateway 1</option>
<option value='option2'>Gateway 2</option>
<option value='option3'>Gateway 3</option>
</select>
When an option element is clicked I want to display a little description for the element. Any ideas how to do that?

You're going to want to use jQuery's change event. I am displaying the text of your option as an alert, but you can display whatever you want based on your needs. (You can also, obviously, put it inside another part of the page...it doesn't need to be an alert.)
$('#myOptions').change(function() {
var val = $("#myOptions option:selected").text();
alert(val);
});
Also, note, that I added an ID to your select tag so that you can more easily handle events to it (I called it myOptions).
Example: http://jsfiddle.net/S9WQv/

As specified by JasCav using jQuery you can accomplish the same in javascript using
<select onchange="alert(this.options[this.selectedIndex].text);">
<option value='option1'>Gateway 1</option>
<option value='option2'>Gateway 2</option>
<option value='option3'>Gateway 3</option>
</select>
Alternatively, onclick event of option, but note that it is not compatible on all browsers.
<select>
<option value='option1' onclick="alert(this.value);" >Gateway 1</option>
<option value='option2'>Gateway 2</option>
<option value='option3'>Gateway 3</option>
</select>

You can write it as below mention using javascript.
document.getElementById("select").addEventListener('change', (event) => {
console.log(event.target.value)
});
<select id="select">
<option value='option1'>Gateway 1</option>
<option value='option2'>Gateway 2</option>
<option value='option3'>Gateway 3</option>
</select>