How can I check whether a option already exist in select by JQuery

This evaluates to true if it already exists:

$("#yourSelect option[value='yourValue']").length > 0;

Another way using jQuery:

var exists = false; 
$('#yourSelect  option').each(function(){
  if (this.value == yourValue) {
    exists = true;
  }
});

if ( $("#your_select_id option[value=<enter_value_here>]").length == 0 ){
  alert("option doesn't exist!");
}

Although most of other answers worked for me I used .find():

if ($("#yourSelect").find('option[value="value"]').length === 0){
    ...
}

It's help me :

  var key = 'Hallo';

   if ( $("#chosen_b option[value='"+key+"']").length == 0 ){
   alert("option not exist!");

    $('#chosen_b').append("<option value='"+key+"'>"+key+"</option>");
    $('#chosen_b').val(key);
   $('#chosen_b').trigger("chosen:updated");
                         }
  });

I had a similar issue. Rather than run the search through the dom every time though the loop for the select control I saved the jquery select element in a variable and did this:

function isValueInSelect($select, data_value){
    return $($select).children('option').map(function(index, opt){
        return opt.value;
    }).get().includes(data_value);
}