Strange JavaScript behaviour, variable changing value before converter function is called

I'm passing down some JSON data from Smarty. I'm applying this to a JavaScript variable, options.

If you've seen my previous question about date formats you'll know I need to do a bit of work on the data coming in, so I've got a function called chart_convert_dates() that's called, passing in the options (well, options.data), and upon it's return setting it back again.

If you read through my code you开发者_JAVA技巧'll notice I'm debugging the options variable, and it changes from the original before the function is called!?

If I comment out the function call, the variable is untouched, as it should be at that point.

This happens with Chrome, FF... what's going on?

{literal}

    <script type="text/javascript">

        $(document).ready(function() {

            // set JS var, this data is coming in from smarty
            var options = {/literal}{$options}{literal};

            // these should both be exactly the same
            debug.debug({/literal}{$options}{literal});
            debug.debug(options);

            // but the above outputs aren't the same! options has been modified
            // by the function below... that hasn't even fired yet!? We can prove
            // this by commenting out the following function call

            options.data = chart_convert_dates(options.data);

            // ... do something else

        });

    </script>

{/literal}

This is, of course, impossible.

You'll probably find that the debug.debug() function is saving a reference to the object it is provided with, rather than converting it to a string immediately. When you then view the contents of it's argument at a later time, the output will reflect the current state of the object, rather than the state it was in.

This is best explained with the following example:

var debug = {
    report: function () {
        // console.log(this._value);
    },
    debug: function (arg) {
        this._value = arg; // save a reference
    }
}

var options = {
    foo: 1
};

debug.debug(options);

options.foo = 2;

debug.report(); // will show 2 (http://jsfiddle.net/zQFPm/)

http://jsfiddle.net/zQFPm/