Why can't I use setTimeout in a javascript object?

Message = function () {



    ...

    ...        



    this.messageFactory = ...

    this.feedbackTag = document.getElementById('feedbackMessages');



    this.addInfo = function (message) {

        var info = this.messageFactory.createInfo(message); // create a div

        this.feedbackTag.appendChild(info);



        setTimeout('this.feedbackTag.removeChild(info)', 5000);

        // why in here, it complain this.feedbacktag is undefined ??????



    };

}

Thanks for Steve`s Solution, now it will work if the code is as below...
because the 'this' before was actually pointing to the function within setTimeOut, it cannot rearch Message.

Message = function () {



    ...

    ...        



    this.messageFactory = ...

    this.feedbackTag = document.getElementById('feedbackMessages');



    this.addInfo = function (message) {

        var info = this.messageFactory.createInfo(message); // create a div

        this.feedbackTag.appendChild(info);



        var _this = this;

        setTimeout(function() { _this.feedbackTag.removeChild(info); }, 5000);



    };

}

But why doesn`t it work if we do this:

Message = function () {



    ...

    ...        



    this.messageFactory = ...

    this.feedbackTag = document.getElementById('feedbackMessages');

    // public function

    this.addInfo = function (message) {

        var info = this.messageFactory.createInfo(message); // create a div

        this.feedbackTag.appendChild(info);



        delayRemove(info);



    };

    // private function

    function delayRemove(obj) {

        var _this = this;

        setTimeout(function() { _this.feedbackTag.removeChild(info); }, 5000);

    }

}