Check if value is in select list with JQuery

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

Use the Attribute Equals Selector

var thevalue = 'foo';
var exists = 0 != $('#select-box option[value='+thevalue+']').length;

If the option's value was set via Javascript, that will not work. In this case we can do the following:

var exists = false;
$('#select-box option').each(function(){
    if (this.value == 'bar') {
        exists = true;
        return false;
    }
});

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;
   }