Question

I wrote this code for add attr to option element that have defined value on index variable:

$(document).ready(function (){

    $('option').each(function() {
      var index = '1';
      if($(this).attr('value') == index)
        $(this).attr('selected','selected');
    });

});

How can add attr to each element that have value listed on index variable. Something like this:

      var index = '1,2,5,8,7,9';
      if($(this).attr('value') == index)
...

UPDATE:
This is my html code:

<select name="category[]" multiple="multiple" id="category">
    <option class="level-0" value="1">Jobs</option>
    <option class="level-1" value="24">CSS</option>
    <option class="level-0" value="5">Products</option>
</select>

Answer 1

$('#category option').each(function() {
  var index = [1,2,5,8,7,9],
      value = parseInt( this.value, 10); // convert the value to integer
  if($.inArray(value, index) >= 0)
    $(this).attr('selected','selected'); //or, $(this).prop('selected', true);
});

Working sample

---

Without array

$('#category option').each(function() {
  var index = '1,2,5,8,7,9',
      value = this.value;
  if(index.indexOf(value) >= 0)
    $(this).attr('selected','selected'); //or, $(this).prop('selected', true);
});

Working sample

---

Using filter()

var index = '1,2,5,8,7,9';
$('#category option').filter(function() {
  return index.indexOf(this.value) >= 0;
}).attr('selected', 'selected');

Working sample

---

Using .attr('selected', callback)

var index = '1,2,5,8,7,9';
$('#category option').attr('selected', function(i, val) {
  if( index.indexOf( this.value) >= 0 )
  return 'selected';
})

Working sample