After Effects Expression (if layer is a CompItem)

Question

I am trying to make slight modification at line 5 of below After Effects expression. Line 5 checks if the layer is visible and active but I have tried to add an extra check that the layer should not be a comp item. (In my project, layers are either text or image layer and I beileve an image layer means a comp item). Somehow the 'instanceof' method to ensure that layer should not be a comp item is not working. Please advise how to fix this error, thanks.

   txt = "";
   for (i = 1; i <= thisComp.numLayers; i++){
      if (i == index) continue;
         L = thisComp.layer(i);
           if ((L.hasVideo && L.active) && !(thisComp.layer(i) instanceof CompItem)){
          txt = i + " / " + thisComp.numLayers + " / " + L.text.sourceText.split(" ").length;
       break;
       }
    }
    txt

Answer 1

While compItem is available only in ExtendScript, you can actually check the properties available in the {my_layer}.source object.

Here's a working example (AE CC2018, CC2019 & CC2020): layer_is_comp.aep

The expression would be something similar to:

function isComp (layer)
{
  try
  {
    /*
    - used for when the layer doesn't have a ['source'] key or layer.source doesn't have a ['numLayers'] key
    - ['numLayers'] is an object key available only for comp objects so it's ok to check against it
    - if ['numLayers'] is not found the expression will throw an error hence the try-catch
    */
    if (layer.source.numLayers) return true;
    else return false;
  }
  catch (e)
  {
    return false;
  }
}

try
{
  // prevent an error when no layer is selected
  isComp(effect("Layer Control")(1)) ? 'yes' : 'no';
}
catch (e)
{
  'please select a layer';
}

For your second question, you can check if a layer is a TextLayer by verifying that it has the text.sourceText property.

Example:

function isTextLayer (layer)
{
  try
  {
    /*
    - prevent an expression error if the ['text'] object property is not found
    */
    var dummyVar = layer.text.sourceText;

    return true;
  }
  catch (e)
  {
    return false;
  }
}