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function addLeadingZeros (n, length){
    var str = (n > 0 ? n : -n) + "";
    var zeros = "";
    for (var i = length - str.length; i > 0; i--)
        zeros += "0";
    zeros += str;
    return n >= 0 ? zeros : "-" + zeros;
}


var docRef = activeDocument;    
var sel = docRef.selection;    
for (var i=0; i < sel.length; i++){ 
    sel[i].name = "obj_" + addLeadingZeros(i, 4);  
};

app.redraw();