[danik.js]

<input type="password" id="old_pass" name="old_pass" />
<input type="password" id="pass" name="pass" onblur="return;CheckReg('pass',this.value)" />
<input type="checkbox" id="show_pass" />
<script src="//code.jquery.com/jquery-latest.js"></script>
<script>
(function(){
    var fields = [$('#old_pass').get(0), $('#pass').get(0)];
    $('#show_pass').change(function() {
        var type = this.checked ? 'text' : 'password';
        try {
            $(fields).prop('type', type);
        } catch (e) {
            // IE workaround
            for (var i=0; i < fields.length; i++) {
                $(fields[i]).replaceWith(function(){
                    return fields[i] = $('<input/>', {
                        id: this.id,
                        name: this.name,
                        value: this.value,
                        onblur: this.onblur,
                        type: type
                    }).get(0);
                });
            }
        }
    });
})();
</script>