How can I batch turn on/turn off layers in a large group of files

Question

I lifted this code from another discussion that purports to say it works, but it doesn't.
I keep getting the Error: Object does not support the property or method 'length'
I've looked at other batch process scripts that that line of code seems to be correct.
I know this is a very simple script, and that the turn on/turn off layer part of the script works perfectly fine (on individuall pages), but for some reason I can't get it to batch process.
I have a multi-version set of files with layers such as "de" for germany, "us" for United States, etc. that I need to batch turn on so that I can print.
Here is what I have:
var myFolder = Folder.selectDialog("Select Indesign Folder");
var myIndsnFiles = myFolder.getFiles("*.indd");
for(k=0; k<myIndsnFiles.length; k++)
   var myIndsnFiles = app.open(myIndsnFiles[k]);
var myIndsnFiles = app.documents[0];
var answersLayer = myIndsnFiles.layers.item("us");
answersLayer.visible = false;
    app.activeDocument.close(SaveOptions.yes);
Can you fix/explain what I'm doing wrong?

Answer 1

I got Sergey's Version to work. See below. It's very different than my original. I understood why my original wasn't working, but couldn't figure out how to fix it. I tried inserting Laubender's code into this, but I got the same error as above "Object does not support the property or method 'length'".
I made some adjustments so that the script turns off all layers that should be off, then turns on the layer I need.
Everything runs perfect except I wonder if there's a way I can turn off any warnings, such as missing links, so that it processes the whole batch without pausing. Maybe that's what Laubender was referring to.
var myFolder = Folder.selectDialog("Select Indesign Folder");
if( myFolder != null )
    var myIndsnFiles = myFolder.getFiles("*.indd");
    for( var k = myIndsnFiles.length -1; k >= 0; k--)
        try
            var myDoc = app.open(myIndsnFiles[k]);
var inLayer = myDoc.layers.item("in");
inLayer.visible = false;
var itLayer = myDoc.layers.item("it");
itLayer.visible = false;
var deLayer = myDoc.layers.item("de");
deLayer.visible = false;
var ukLayer = myDoc.layers.item("uk");
ukLayer.visible = false;
var usLayer = myDoc.layers.item("us");
usLayer.visible = false;
var intLayer = myDoc.layers.item("INT");
intLayer.visible = false;
deLayer.visible = true;
            myDoc.close(SaveOptions.yes);
        } catch ( error ) {};// try-catch
    };// for
};// if