Select value in tab

The JavaScript below works perfectly. But when I change tab it doesn’t work! Thank you very much in advance.

        <button id="toggle" name="test" onclick="myFunction()">on</button>

        <select id="help-comradeso[]">
    <option value="1">Janeiro</option>                           
    <option value="2">Fevereiro</option>                           
    <option value="3">Mar?o</option>                           
    <option value="4">Abril</option>                           
    <option value="5">Maio</option>                           
    <option value="6">Junho</option>                           
    <option value="7">Julho</option>                           
    <option value="8">Agosto</option>                           
    <option value="1912">Setembro</option>                           
    <option value="10">Outubro</option>                           
    <option value="11">Novembro</option>                           
    <option value="12">Dezembro</option>                           
</select>

        <script>

            var mes = 8;
document.getElementById('help-comradeso[]').value = mes;

        </script>
    </body>
</html>