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How do I best access the value attribute of the DOM-Element with jquery?

I have dynamically generated the following dropdown list using jquery for the calculator app I am currently making:
<select type="text" id="field__left">
<option label="Please choose an unit!" text="Please choose an unit!"></option>
<option label="inch" text="Inch" value="0.0254"></option>
<option label="foot" text="Foot" value="0.3048"></option>
<option label="yard" text="Yard" value="0.9144"></option>
<option label="rod" text="Rod" value="5.0292"></option>
<option label="chain" text="Chain" value="20.1168"></option>
<option label="furlong" text="Furlong" value="201.168"></option>
<option label="mile" text="Mile" value="1609.344"></option>
<option label="cable" text="Cable" value="185.2"></option>
<option label="nautical mile" text="Nautical mile" value="1852"></option>
<option label="shipday" text="Shipday" value="185200"></option>
</select>
What now I try is to access the value attribute of every option, but I don't get far. The examiner is showing the value attribute in the elements tab, I can also find under the options when I look at the properties in the browser, but I am unable to access them via JavaScript.
I tried:
const leftVal = $('#field__left').children('option').attr('value');
also
const leftVal = $('#field__left').children('option').data('value');
but it returned undefined, while:
const leftVal = document.querySelector('#field__left').getAttribute('value');
gave me null.
Anybody has the ide where my mistake lies?
Thank you in advance.

I try is to access the value attribute of every option
You need a loop...
$("#field__left option").each(function(){
console.log($(this).val())
})
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<select type="text" id="field__left">
<option label="Please choose an unit!" text="Please choose an unit!"></option>
<option label="inch" text="Inch" value="0.0254"></option>
<option label="foot" text="Foot" value="0.3048"></option>
<option label="yard" text="Yard" value="0.9144"></option>
<option label="rod" text="Rod" value="5.0292"></option>
<option label="chain" text="Chain" value="20.1168"></option>
<option label="furlong" text="Furlong" value="201.168"></option>
<option label="mile" text="Mile" value="1609.344"></option>
<option label="cable" text="Cable" value="185.2"></option>
<option label="nautical mile" text="Nautical mile" value="1852"></option>
<option label="shipday" text="Shipday" value="185200"></option>
</select>

$('#field__left').children('option') will return an array of all select option nodes, you have to iterate through it to get values of each option
Here is the simple solution
// get list of an options
var options = $('#field__left option');
// Then, convert that into an array of just the values
var values = $.map(options, e => $(e).val())
console.log(values)

How to show/hide items from a select list based on another selection

In JavaScript, how to show/hide items from a select list based on another selected option that contain a certain word or words?
I am very new to JavaScript, so any help would be appreciated.
There are two drop-downs: "group" and "alph".
<select name="group">
<option value="Angry (Two)">Angry (Two)</option>
<option value="Happy (Two)">Happy (Two)</option>
<option value="Sad">Sad</option>
<option value="Tired (One)">Tired (One)</option>
</select>
<select name="alph">
<option value="ABC">ABC</option>
<option value="ABC-1">ABC-1</option>
<option value="ABC-2">ABC-2</option>
<option value="DEF">DEF</option>
<option value="DEF-1">DEF-1</option>
<option value="DEF-2">DEF-2</option>
<option value="DEF-3">DEF-3</option>
</select>
Without IDs, for the first dropdown (name group), if the selected value contains " (Two)", then the list will only show:
<select name="alph">
<option value="ABC-2">ABC-2</option>
<option value="DEF-2">DEF-2</option>
</select>
If the user changes selection, and the selected value contains " (One)", then the list will only show:
<select name="alph">
<option value="ABC-1">ABC-1</option>
<option value="DEF-1">DEF-1</option>
</select>
If the user changes selection, and the selected value does not contain neither " (One)" or " (Two)", then the list will show:
<select name="alph">
<option value="ABC">ABC</option>
<option value="DEF">DEF</option>
<option value="DEF-3">DEF-3</option>
</select>
Note: I am not able to add IDs or attributes. I can only access the name of the select and the value.

Use data-* to create groups, then use a querySelector to get all the options and test if they are apart of the group or not. If they are apart of the group show them otherwise hide them.
// The main group
let groups = document.querySelector('select[name=group]')
// Add a change event
groups.addEventListener('change', (e) => {
// Get the currently selected Group
let current = e.target.options[e.target.selectedIndex]
// Get the data-group number
let group = current.getAttribute('data-group')
// Get all the items from the second dropdown
let opts = Array.from(document.querySelectorAll('select[name=alph]>option'))
// Hide items that are not appart of the group
opts.forEach(itm => itm.style.display = itm.getAttribute('data-group') == group || !itm.getAttribute('data-group') ? 'initial' : 'none')
// Reset the the selection
document.querySelector('select[name=alph]').selectedIndex = 0
})
/* Hide option two items by default */
select[name=alph]>option[data-group]{display:none;}
<select name="group">
<option>Select One...</option>
<option data-group="1" value="Angry (Two)">Angry (Two)</option>
<option data-group="2" value="Happy (Two)">Happy (Two)</option>
<option data-group="3" value="Sad">Sad</option>
<option data-group="4" value="Tired (One)">Tired (One)</option>
</select>
<select name="alph">
<option>Select One...</option>
<option data-group="1" value="ABC">ABC</option>
<option data-group="1" value="ABC-1">ABC-1</option>
<option data-group="2" value="ABC-2">ABC-2</option>
<option data-group="2" value="DEF">DEF</option>
<option data-group="3" value="DEF-1">DEF-1</option>
<option data-group="3" value="DEF-2">DEF-2</option>
<option data-group="4" value="DEF-3">DEF-3</option>
</select>

Php: modify attribute html tag

I have these html tags:
<select name="ct" id="ct">
<option value="-1">Tutte</option>
<option value="1">Da verificare</option>
<option value="2">Verificate</option>
<option value="3">Approvate</option>
<option value="4">Respinte</option>
<option value="5">Pubblicate</option>
<option value="6">Scadute</option>
<option value="7">Proposte</option>
<option value="8">Rifiutate</option>
<option value="9">Ritirate</option>
<option selected="selected" value="7,8,1">Proposte / Rifiutate / Da verificare</option>
</select>
I want to change the attribute value of the selected option (which contains "7,8,1") to value="-1", so it will look like:
<option selected="selected" value="-1">Proposte / Rifiutate / Da verificare</option>
I tried with:
$dom_richieste->getElementsByTagName('options')->getAttribute('value').value="-1";
But that's not working...

You can use $("#ct option[value='7,8,1']").val("-1");
$("#ct option[value='7,8,1']").val("-1");
console.log($("#ct").val());
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<select name="ct" id="ct">
<option value="-1">Tutte</option>
<option value="1">Da verificare</option>
<option value="2">Verificate</option>
<option value="3">Approvate</option>
<option value="4">Respinte</option>
<option value="5">Pubblicate</option>
<option value="6">Scadute</option>
<option value="7">Proposte</option>
<option value="8">Rifiutate</option>
<option value="9">Ritirate</option>
<option selected="selected" value="7,8,1">Proposte / Rifiutate / Da verificare</option>
</select>
I suggest you to check how you created this drop down, and if possible replace there instead after creating drop down.

If I understand correctly you are need something like:
foreach ($dom->getElementsByTagName('option') as $item) {
if ($item->getAttribute('selected') == "selected")
$item->setAttribute("value", "-1");
}
This way you pass on your option items and set the value of the selected ones

You have used getElementsByTagName('options') in your code and no tag available with name options. Instead you should use correct tag name option.

How to copy and paste an html select selected value to other selects

I have developed a site with multiple select elements in the same form
<select>
<option value="">Select ...</option>
<option value="1">Apple</option>
<option value="2">Banana</option>
<option value="3">Carrot</option>
<option value="4">Orange</option>
<option value="5">Pear</option>
</select>
There are 8 selects with the exact same options on the page.
The user wants to copy and paste the selected values of a select from one to another.
The end user initiates it by using Ctrl+C, Ctrl+V. The web app was written to replace an excel app which allows copy and paste by the end user
However an html select doesn't support copy and paste. (Ctrl+C, Ctrl+V)
What can I do?
Any plugin that can maybe do this? Any minimal autocomplete select to suggest that looks like the standard select?
Edit:
Using the datalist example can be a solution. Only problem is that it allows invalid text i.e. text that is not one of the select options (apart from nothing) to be typed in. How do I only allow valid text?

See #Hashbrown answer in Copy selected item's text from select control html question. Sure it would help you as it's partially match your question.
Also you can take a look on How do I copy to the clipboard in JavaScript? as it has a lot of info related to your question.
Good Luck!

You should use datalist and handle none valid input as follow:
$('input[list]').on('change', function() {
var options = $('option', this.list).map(function() {
return this.value
}).get();
if (options.indexOf(this.value) === -1) {
this.value = "";
}
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<input list="food1" name="food1" placeholder="Select...">
<datalist id='food1'>
<option disabled>Select ...</option>
<option value="Apple">Apple</option>
<option value="Banana">Banana</option>
<option value="Carrot">Carrot</option>
<option value="Orange">Orange</option>
<option value="Pear">Pear</option>
</datalist>
<input list="food2" name="food2" placeholder="Select...">
<datalist id='food2'>
<option value="Apple">Apple</option>
<option value="Banana">Banana</option>
<option value="Carrot">Carrot</option>
<option value="Orange">Orange</option>
<option value="Pear">Pear</option>
</datalist>

Try using datalist
<input list="foods" name="food">
<datalist id='food'>
<option value="1">Select ...</option>
<option value="1">Apple</option>
<option value="2">Banana</option>
<option value="3">Carrot</option>
<option value="4">Orange</option>
<option value="5">Pear</option>
</datalist>
http://www.w3schools.com/tags/tag_datalist.asp

Set the "value" attribute of one select to that of another with
document.getElementById('[select element1]').value = document.getElementById('[select element2]').value;
You can put this in the onClick handler for a button if you want.

Try this code
var src=document.getElementById('src');
var des=document.getElementById("des");
var opt=document.createElement("Option");
opt.value = src.options[src.selectedIndex].value;
opt.text = src.options[src.selectedIndex].text;
des.options.add(opt);

See the datalist element: http://www.w3schools.com/tags/tag_datalist.asp
It is used like this:
<input list="browsers" name="browserselect" type="text"/>
<datalist id="browsers">
<option value="Internet Explorer">
<option value="Firefox">
<option value="Chrome">
<option value="Opera">
<option value="Safari">
</datalist>
The list attribute of the input element tells the input element to use the datalist with that id.

<select name="a" id="a">
<option value="1">abc</option>
</select>
<select name="b" id="b">
<option value=""></option>
</select>
<script type="text/javascript">
$('select#a').on('change',function(){
var v = $(this).val();
$('select#b').val(v);
});
</script>

Html and javascript <select> tags with filtering

I will try to be as specific as I can.
So I need to create a selector that filters the options.
I have:
types of cars
The places where you can drive those cars.
So for example you would select > Ferrari > then the correct areas on the second select tag show up
I.e. Select > Ferrari = London, Cambridge, Devon, New hampshire
Select > Lamborghini = London, Bertshire, Oakwood,
And then finally
they choose (Ferrari + Cambridge) and then press "go" and jump to the final link that will take them to the right page.
My code is:
<script type='text/javascript' src='http://code.jquery.com/jquery-1.6.js'></script>
<script type='text/javascript'>//<![CDATA[
$('#filter-regions').on('click', function() {
var pilotage-carFilter = $('#pilotage-car').text();
var itemFilter = $('#items').text();
console.log('pilotage-carFilter: ' + pilotage-carFilter);
console.log('itemFilter : ' + itemFilter);
console.log('Applying filter now...');
featureList.filter(function(item) {
console.log('Running filter() on item: ('+item+')');
console.log('item.values().pilotage-car: ' + item.values().pilotage-car);
console.log('item.values().item: ' + item.values().item);
return
(pilotage-carFilter==='Ferrari' || item.values().pilotage-car === pilotage-carFilter)
&& (itemFilter==='All items' || item.values().item === itemFilter);
});
return false;
});
//]]>
</script>
</head>
<body>
<form id="filter">
<select id="pilotage-car" name="pilotage-car" size="1">
<option value="http://www.coolcadeau.fr/Stage-de-pilotage-Ferrari-BN-5iZ5.aspx?SqNo=5iZ5&cm_sp=LHN-_-Voiture-_-Ferrari&cm_re=Ferrari-_-Voiture-_-LHN">Ferrari</option>
<option value="Porsche">Porsche</option>
<option value="Lamborghini" selected>Lamborghini</option>
<option value="Mustang">Mustang</option>
<option value="Audi" selected>Audi</option>
<option value="Multivolants">Multivolants</option>
<option value="Rallye">Rallye</option>
<option value="Subaru">Subaru</option>
<option value="Karting">Karting</option>
<option value="4x4">4x4</option>
<option value="Moto">Moto</option>
<option value="Quad">Quad</option>
<option value="Buggy">Buggy</option>
<option value="Renault Sport">Renault Sport</option>
<option value="Prototype">Prototype</option>
<option value="Chevrolet">Chevrolet</option>
<option value="Corvette">Corvette</option>
</select>
<select id="items" name="items" size="1">
<option value="http://www.coolcadeau.fr/Stage-de-pilotage-Ferrari-Alsace-BN-5iZ5Z1z13skq.aspx?SqNo=5iZ5Z1z13skq&cm_sp=LHN-_-Region-_-Alsace&cm_re=Alsace-_-Region">A l'étranger</option>
<option value="Alsace">Alsace</option>
<option value="Aquitaine" selected>Aquitaine</option>
<option value="Auvergne">Auvergne</option>
<option value="Basse-Normandie" selected>Basse-Normandie</option>
<option value="Bourgogne">Bourgogne</option>
<option value="Bretagne">Bretagne</option>
<option value="Centre">Centre</option>
<option value="Champagne-Ardenne">Champagne-Ardenne</option>
<option value="Franche-Comté">Franche-Comté</option>
<option value="Haute-Normandie">Haute-Normandie</option>
<option value="Ile-de-France">Ile-de-France</option>
<option value="Languedoc-Roussillon">Languedoc-Roussillon</option>
<option value="Limousin">Limousin</option>
<option value="Lorraine">Lorraine</option>
<option value="Midi-Pyrénées">Midi-Pyrénées</option>
<option value="Nord-Pas-de-Calais">Nord-Pas-de-Calais</option>
<option value="Pays de la Loire">Pays de la Loire</option>
<option value="Picardie">Picardie</option>
<option value="Poitou-Charentes">Poitou-Charentes</option>
<option value="Provence-Alpes-Côte d'Azur">Provence-Alpes-Côte d'Azur</option>
<option value="Rhône-Alpes">Rhône-Alpes</option>
</select>
<input id="go-button" type="button" name="test" value="Go"/>
</form>
Im really not sure about javascript at all.
How could I, and/or what is the best way I could achieve this? Are there any examples out there?
EDIT: I found something like this http://jsfiddle.net/dtAgX/1/
But i need a sumbit button that will link to the right page based on selection.
Thanks in advance

You have several options depending on your backend, PHP, ASP ect. or plain HTML.
If you use PHP/ASP you can provide an 'action' and a 'method' for your and have the server provide the correct page, depending on the values of your selects.
If you use plain HTML, you can add an 'onsubmit' event to your and have a javascript redirect to the correct page.
In both cases the "go-button" should be of type "submit".
If you want more help I'll need to know more about your setup :-)
Also, can all cars be driven at all locations?