detect-private-browsing.js

## detect-private-browsing.js
function retry(isDone, next) {
    var current_trial = 0, max_retry = 50, interval = 10, is_timeout = false;
    var id = window.setInterval(
        function() {
            if (isDone()) {
                window.clearInterval(id);
                next(is_timeout);
            }
            if (current_trial++ > max_retry) {
                window.clearInterval(id);
                is_timeout = true;
                next(is_timeout);
            }
        },
        10
    );
}

function isIE10OrLater(user_agent) {
    var ua = user_agent.toLowerCase();
    if (ua.indexOf('msie') === 0 && ua.indexOf('trident') === 0) {
        return false;
    }
    var match = /(?:msie|rv:)\s?([\d\.]+)/.exec(ua);
    if (match && parseInt(match[1], 10) >= 10) {
        return true;
    }
    return false;
}

function detectPrivateMode(callback) {
    var is_private;

    if (window.webkitRequestFileSystem) {
        window.webkitRequestFileSystem(
            window.TEMPORARY, 1,
            function() {
                is_private = false;
            },
            function(e) {
                console.log(e);
                is_private = true;
            }
        );
    } else if (window.indexedDB && /Firefox/.test(window.navigator.userAgent)) {
        var db;
        try {
            db = window.indexedDB.open('test');
        } catch(e) {
            is_private = true;
        }

        if (typeof is_private === 'undefined') {
            retry(
                function isDone() {
                    return db.readyState === 'done' ? true : false;
                },
                function next(is_timeout) {
                    if (!is_timeout) {
                        is_private = db.result ? false : true;
                    }
                }
            );
        }
    } else if (isIE10OrLater(window.navigator.userAgent)) {
        is_private = false;
        try {
            if (!window.indexedDB) {
                is_private = true;
            }
        } catch (e) {
            is_private = true;
        }
    } else if (window.localStorage && /Safari/.test(window.navigator.userAgent)) {
        try {
            window.localStorage.setItem('test', 1);
        } catch(e) {
            is_private = true;
        }

        if (typeof is_private === 'undefined') {
            is_private = false;
            window.localStorage.removeItem('test');
        }
    }

    retry(
        function isDone() {
            return typeof is_private !== 'undefined' ? true : false;
        },
        function next(is_timeout) {
            callback(is_private);
        }
    );
}

## example.html
<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
</head>
<body>
  <div id="result"></div>
  <script src="detect-private-browsing.js"></script>
  <script>
    detectPrivateMode(
        function(is_private) {
            document.getElementById('result').innerHTML = typeof is_private === 'undefined' ? 'cannot detect' : is_private ? 'private' : 'not private';
        }
    );
  </script>
</body>
</html>