How to set ruler 0 point to canvas?

Thank you for sharing this info. This was very usefull. 

I did some small adjustments. Below my working code...

var myDoc = app.activeDocument;

var dim = getLargestBounds(); 

myDoc.rulerOrigin = [dim[0],dim[1]+myDoc.height];


function getLargestBounds() {  
    var tempLayer, tempText, left, top, LARGEST_SIZE = 16383;  

    tempLayer = app.activeDocument.layers.add();  
    tempText = tempLayer.textFrames.add();  

    left = tempText.matrix.mValueTX;  
    top = tempText.matrix.mValueTY;  

    tempLayer.remove();  

    var dimensions = [left, top, left + LARGEST_SIZE, top + LARGEST_SIZE ];

    return dimensions;
}