Making textfield as required using javascript

Visualforce Page:

<apex:page id="page">
<script>
    function req(){
    var n=document.getElementById('page:form:pb:name').value;
     if(n=='')
     alert('Please enter name. Because it is mandatory field');
    
                  }
</script>
    <apex:form id="form">
        <apex:pageBlock id="pb">
           
            Name:<apex:inputtext id="name"/>
            <apex:commandButton value="Enter" onclick="req()"/>
          
        </apex:pageBlock>
    </apex:form>
</apex:page>