Issue with javascript and [play~] object

I have been working on a solution based on the [play~] object. However I have had challenges with "clipping" so I have designed a solution where the aim was to generate an envelope at the beginning and at the end of the playback of the segment. However I have not been able to get this to work using javascript and clipping is still occuring.

Here is the code:
// inlets and outlets
inlets = 1;
outlets = 1;

var tskOneFragment;

function playOneFragment(){

    line1= this.patcher.getnamed("sn_line1");
    play1 = this.patcher.getnamed("sn_play1");

    tskOneFragment = new Task(fade, this);

    //----Set line object to 0
    line1.message(0,0);

    //----start playback of fragment (play from 300ms to 800ms
    //and use 500ms on the operation)
    play1.message("start",300,800,500);
    //increase envelope to 1 using 20ms on the operation
    //playback should have started giving a nice fade up
    line1.message(1,20);

    //----Fade before the fragment ends
    tskOneFragment.interval = 450;
    tskOneFragment.repeat(2);

}

function fade(){
    var iter= arguments.callee.task.iterations;
    //Need to use iter=2 because task first run is immediate
    if(iter==2){
        //Fade clip should end playback 10ms before fragment playback ends
        line1.message(0,40);
    }

}