This requires you to serialize the javascript array into a string, something that can easily be done using the JSON object.

var myArray = [1, 2, 3];

var myJson = JSON.stringify(myArray); // [1,2,3]

....

xhr.send({

    data:{

        param: myJson

    }

});

As the JSON object is not present in older browsers you should include Douglas Crockfords json2 library

If you already rely on some library that includes methods for encoding/serializing then you can use this instead. E.g. ExtJs has Ext.encode