Hi there, I new with AE script (I much familiar with ActionScript in Flash, not an expert though). I try to control my solid object using GUI Slider I made but it's doesn't work. I need help here. Here is my code:

Question

myComp = app.project.item(1);
mySolid = myComp.layers.addSolid([1.0,1.0,0], "my square", 50, 50, 1);
var myWin = new Window("palette", "Transform", undefined);
    myWin.orientation = "row";
var myOpacity = myWin.add("panel", undefined, "Opacity");
    myOpacity.orientation = "column";
    myOpacityText = myOpacity.add("statictext", [290, 20, 380, 40], "Current value:");
    myOpacityDisplay = myOpacity.add("statictext", [385, 20, 425, 40], "100");
    myOpacity.add("statictext", undefined, "Kontrol Opacity");
    theOpacity = myOpacity.add("slider", [20, 20, 230, 45], 50, 0, 100);
    var layerOpacity = mySolid.property("opacity");
    theOpacity.onChanging = function()
         var val = Math.round(theOpacity.value);
  myOpacityDisplay.text = val;
         layerOpacity.value = val;
myWin.center();
myWin.show();

Answer 1

I don't write scripts that do this because the features are already available with Expression Controls and expressions. That's the route I would suggest. You are reinventing the wheel. If you want specific help on scripting then try asking some questions in the scripting sub forum for AE. You'll find the forum under sub communities.
If you just want to create a slider for opacity that you can add to any layer you select first add an expression control slider to the layer, then press t to reveal the opacity property, then hold down the Alt/Option key and click on the stopwatch to start and expression, then drag the pickwhip to the slider control. Now, with transparency selected select Animation>Save Animation Preset and give it a name. Now when you need a slider to control a layer, or to control a bunch of layers simply select them and apply your preset.
You could also put your expression control slider on a null, give the name a null, apply the expression to a layer and then save the transparency property as an animation preset. Now if you have 50 layers you want to control with a single slider just add the null, change the name, then select all 50 layers and apply your animation preset.
I have a library of about 200 custom animation presets that use expressions to do most of my repetitive  tasks. It saves a ton of time. Heres one that fades a layer in and then out based on the in and out point of the layer. I use this one daily. You'll see what I mean if you apply it to a layer. fadeIO.ffx