Get values of form field in javascript from a visualforce page

<apex:page >

    <apex:form id="myForm1">

        <apex:pageblock id="pb1">

            <apex:pageblockSection id="pbs1">

                <apex:inputCheckbox id="checkbox1" label="Check Me" onclick="javascript:getCheckBoxValue();">

                </apex:inputCheckbox>

            </apex:pageblockSection>

        </apex:pageblock>

    </apex:form>

    <script language="javascript">

        function getCheckBoxValue() {

            alert(document.getElementById('{!$Component.myForm1.pb1.pbs1.checkbox1}').checked);   

        }

    </script>

</apex:page>