JavaScript String indexOf() selects also 1&3 instead of only 13

I have this indexOf(), the issue I have is that when norms[] has an dataset like 2,13 that options that are set as selected in norm_id['+nr+'][] are not only the values 2 and 13 but also the values 1 and 3

var element = document.getElementById('norm_id['+nr+'][]');
var values = norms[];
for (var i = 0; i < element.options.length; i++) {
   element.options[i].selected = values.indexOf(element.options[i].value) >= 0;
}

Any suggestions how to solve this?

Answer

I have solved the issue with the code below.

var element = document.getElementById('calc_norm_id['+nr+'][]');
var values = norms;
var values_split = values.split(',');

for (var i = 0; i < element.options.length; i++)
{
   for(var j = 0; j < values_split.length; j++)
   {
       if(element.options[i].value == values_split[j])
       {
           element.options[i].selected = element.options[i].value;
       }
   }
}

@Andreas thx for the tip