Hi,

Try the below code snippet as reference:

<apex:page id="p1" controller="Test">

 <apex:form id="f1">

<apex:pageBlock id="pb1">

<apex:pageblockSection id="pbs1" >

<apex:selectList id="pl1"  value="{!countries}" multiselect="false" onchange="check()">

            <apex:selectOptions value="{!items}"/>

            </apex:selectList>

</apex:pageblockSection>

</apex:pageBlock>

 </apex:form>

 <script>

 function check()

 {

var x;

  x = (document.getElementById('p1:f1:pb1:pbs1:pl1').value);

  alert(x);

 }

 </script>

</apex:page>

public class Test {

 String[] countries = new String[]{};

    public String[] getCountries() {

    System.debug('_________________countries___________'+countries);

            return countries;

        }

        public void setCountries(String[] countries) {

            this.countries = countries;

            System.debug('_________________countries__set_________'+countries);

        }

 public List<SelectOption> getItems() {

            List<SelectOption> options = new List<SelectOption>();

            options.add(new SelectOption('US','US'));

            options.add(new SelectOption('CANADA','Canada'));

            options.add(new SelectOption('MEXICO','Mexico'));

             options.add(new SelectOption('CHINA','China'));

            options.add(new SelectOption('LONDON','London'));

            return options;

        }

}

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