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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>

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>

Math formulas from option values

Please help me with this solution. I have simple form:
<select id="select-1" name="select-1">
<option value="0"> option 1</option>
<option value="1"> option 2</option>
</select>
<select id="select-2" name="select-2">
<option value="1"> option 1</option>
<option value="2"> option 2</option>
<option value="3"> option 3</option>
</select>
Below this form i want display dynamically calculation like this:
option from select-1 is in array (e.g. value[0]=1.50, value[1]=5)
option from select-2 is option value from list
In start result: value[0]=1.50 * 1 = 1,50
after change select result will change dynamically.
Angular is neccessary or only JS?

You have to set the value of each option of your select-2 with a "on-change event" on select-1.
In your on-change function you have to get the value of the select-1 and set the different values for your select-2.

Set Display Value in Dropdown List Based on Selection

I have a standard drop down list and I would like it to display the VALUE when closed but the text when expanded for selection. For example, based on my code below, if the user selects 'Item 3' from the list, 3 should be displayed when the list is closed. I'm to the point where I can get the value selected but I don't know how to rewrite what is displayed in the drop down list.
Appreciate any help!
<script type="text/javascript">
function setValue()
{
var value=document.getElementById("mySelect").value;
//Don't know what to put here
}
</script>
<select name="mySelect" id="mySelect" onchange="setValue();">
<option value="1" selected="">Item 1</option>
<option value="2">Item 2</option>
<option value="3">Item 3</option>
<option value="4">Item 4</option>
</select>

Declare the mySelect variable which contains the <select> DOM. Then by using mySelect, you can get the attributes of the <select> tag.
var mySelect = document.getElementById("mySelect");
Then, you can access to the mySelect options which is an array.
mySelect.options[]
mySelect.selectedIndex will give you the current selected index of the tag.
Finally, by appending the .text attribute, you can set the new value of the current selection.
mySelect.options[mySelect.selectedIndex].text = mySelect.value;
Something like this:
function setValue() {
var mySelect = document.getElementById("mySelect");
mySelect.options[mySelect.selectedIndex].text = mySelect.value;
}
<select name="mySelect" id="mySelect" onchange="setValue();">
<option value="1" selected="">Item 1</option>
<option value="2">Item 2</option>
<option value="3">Item 3</option>
<option value="4">Item 4</option>
</select>

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>