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index.html
 
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8">
</head>
<body>
    <div id="output"></div>
    <script>
        var str = 'Hello World. Beautiful World.';
        var searchStr = 'World';
        var index = str.indexOf(searchStr);
        document.getElementById('output').innerHTML = 'Index of first occurrence : ' + index;
    </script>
</body>
</html>