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<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-type" content="text/html;charset=UTF-8">
<title>Watch for a file change</title>
<style type='text/css'>
body {
    font-family: sans-serif;
}
</style>
</head>
<body>
<input type='file' id='filename'>
<input type='button' id='btnStart' value='Start'>
<script type='text/javascript'>
(function() {
    var input;
    var lastMod;
​
    document.getElementById('btnStart').onclick = function() {
        startWatching();
    };
    function startWatching() {
        var file;
​
        if (typeof window.FileReader !== 'function') {
            display("The file API isn't supported on this browser yet.");
            return;
        }
​
        input = document.getElementById('filename');
        if (!input) {
            display("Um, couldn't find the filename element.");
        }
        else if (!input.files) {
            display("This browser doesn't seem to support the `files` property of file inputs.");
        }
        else if (!input.files[0]) {
            display("Please select a file before clicking 'Show Size'");
        }
        else {
            file = input.files[0];
            lastMod = file.lastModifiedDate;
            display("Last modified date: " + lastMod);
            display("Change the file");
            setInterval(tick, 250);
        }
    }
​
    function tick() {
        var file = input.files && input.files[0];
        if (file && lastMod && file.lastModifiedDate.getTime() !== lastMod.getTime()) {
            lastMod = file.lastModifiedDate;
            display("File changed: " + lastMod);
        }
    }
​
    function display(msg) {
        var p = document.createElement('p');
        p.innerHTML = msg;
        document.body.appendChild(p);
    }
})();
</script>
</body>
</html>
​
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