Implementing another funtion to be run “onSongEnd”

Question

A kind person on here has helped me put together a script that:

1: Gets some data from my database

2: Display some information on the screen

3: Get the time a song began began, get the duration of the song in seconds and create a countdown timer to refresh the page

The data comes from MySql Database, its basically a history list that shows what song is currently playing and what has played before it (the metadata contained concists of artist, title, date_played, duration etc....

Now my question is:

How can i add a function to also run when it is time to refresh the playlist

for now i would like to just add a test function to alert("extra function added"); I would expect the screen to alert this message each time there is the PHP request for new data.

I have tried many attempts but im always creating functions and then calling them in the wrong area of the code so i wont bore you experts with the mess.

Thanks for reading... if you could shed some light it would really help me understand correct procedures.

Here is the script.

var PlayList = function (onUpdate, onError)
{
    // store user callbacks
    this.onUpdate = onUpdate; 
    this.onError  = onError;

    // setup internal event handlers
    this.onSongEnd = onSongEnd.bind (this);

    // allocate an Ajax handler
    try
    {
        this.ajax = window.XMLHttpRequest
            ? new XMLHttpRequest()
            : new ActiveXObject("Microsoft.XMLHTTP");
    }
    catch (e)
    {
        // fatal error: could not get an Ajax handler
        this.onError ("could not allocated Ajax handler");
    }
    this.ajax.onreadystatechange = onAjaxUpdate.bind(this);



    // launch initial request

//ADDED CODE
onSongEndWrapper();
//END ADDED CODE

    this.onSongEnd ();

    // ------------------------------------------
    // interface
    // ------------------------------------------

    // try another refresh in the specified amount of seconds
    this.retry = function (delay)
    {
        setTimeout (this.onSongEnd, delay*5000);
    }

    // ------------------------------------------
    // ancillary functions
    // ------------------------------------------
    // called when it's time to refresh the playlist
    function onSongEnd ()
    {
        this.ajax.open('GET', 'playlist.php',
                       true);
        this.ajax.send(null);  
    }

//ADDED CODE
function onSongEndWrapper()
{
   alert("extra function added");
   this.onSongEnd();
}    
//END ADDED CODE



    // handle Ajax request progress
    function onAjaxUpdate ()
    {       
        if (this.ajax.readyState != 4) return;
        if (this.ajax.status == 200)
        {
            // get our response
            var list = eval ('('+this.ajax.responseText+')');
            // compute milliseconds remaining till the end of the current song
            var start = new Date(list.dbdata[0].date_played.replace(' ', 'T')).getTime(); 

            var now   = (list.servertime);
            var d = start - now + 6500
                  + parseInt(list.dbdata[0].duration); 

            if (d < 0)

            {
                // no new song started, retry in 3 seconds
                d = 3000;
            }
            else
            {
                // notify caller
                this.onUpdate (list);
            }

            // schedule next refresh

//ADDED CODE
    setTimeout (onSongEndWrapper.bind(this), d);
//END ADDED CODE

        }
        else
        {
            // Ajax request failed. Most likely a fatal error
            this.onError ("Request failed");
        }       
    }
}

var list = new PlayList (playlistupdate, playlisterror);

function playlistupdate (list)
{
for (var i = 0 ; i != list.dbdata.length ; i++)
    {
        var song = list.dbdata[i];
    }
    {
        $("#list0artist").html(list.dbdata[0].artist);
        $("#list0title").html(list.dbdata[0].title);
    }
}

function playlisterror (msg)
{
    // display error message
    console.log ("Ajax error: "+msg);

    // may want to retry, but chances of success are slim
    list.retry (10); // retry in 10 seconds
}

Answer 1

Create a wrapper function.

setTimeout (onSongEndWrapper.bind(this), d);

function onSongEndWrapper()
{
   alert("extra function added");
   this.onSongEnd();
}

To start on onload add

document.body.addEventListener("load", startPlayList, false);
function startPlayList()
{
   list = new PlayList (playlistupdate, playlisterror);
}

And change this:

var list = eval ('('+this.ajax.responseText+')');

to:

var list = JSON.parse(this.ajax.responseText);