JavaScript alert box, How to Display a Alert Message in JavaScript

it's Very Easy To Display alert box in JavaScript Using JavaScript alert() method

Display JavaScript alert box on Page Load

<script type="text/javascript">
    window.onload=function(){
        alert('JavaScript alert box on Page Load');
    }
</script>

Display JavaScript alert box when user Click on a Link

<script type="text/javascript">

    function javascript_alert(){

        alert("On-click JavaScript Alert");

    }

</script>

<a href="" onclick="javascript_alert();return false">Click to Display Alert Box</a>