[drop-down-menu] Check if value is in select list with JQuery

How can I, using JQuery, check if a value belongs to dropdown list or not?

Having html collection of selects you can check if every select has option with specific value and filter those which don't match condition:

//get select collection:
let selects = $('select')
//reduce if select hasn't at least one option with value 1
.filter(function () { return [...this.children].some(el => el.value == 1) });

Just in case you (or someone else) could be interested in doing it without jQuery:

var exists = false;
for(var i = 0, opts = document.getElementById('select-box').options; i < opts.length; ++i)
   if( opts[i].value === 'bar' )
   {
      exists = true; 
      break;
   }

I know this is kind of an old question by this one works better.

if(!$('.dropdownName[data-dropdown="' + data["item"][i]["name"] + '"] option[value="'+data['item'][i]['id']+'"]')[0]){
  //Doesn't exist, so it isn't a repeat value being added. Go ahead and append.
  $('.dropdownName[data-dropdown="' + data["item"][i]["name"] + '"]').append(option);
}

As you can see in this example, I am searching by unique tags data-dropdown name and the value of the selected option. Of course you don't need these for these to work but I included them so that others could see you can search multi values, etc.

Here is another similar option. In my case, I'm checking values in another box as I build a select list. I kept running into undefined values when I would compare, so I set my check this way:

if ( $("#select-box option[value='" + thevalue + "']").val() === undefined) { //do stuff }

I've no idea if this approach is more expensive.

if ($select.find('option[value=' + val + ']').length) {...}

Why not use a filter?

var thevalue = 'foo';    
var exists = $('#select-box option').filter(function(){ return $(this).val() == thevalue; }).length;

Loose comparisons work because exists > 0 is true, exists == 0 is false, so you can just use

if(exists){
    // it is in the dropdown
}

Or combine it:

if($('#select-box option').filter(function(){ return $(this).val() == thevalue; }).length){
    // found
}

Or where each select dropdown has the select-boxes class this will give you a jquery object of the select(s) which contain the value:

var matched = $('.select-boxes option').filter(function(){ return $(this).val() == thevalue; }).parent();