This script errors during foreach while using illustrator

script error

var destFolder, sourceFolder, files, fileType, sourceDoc, targetFile, epsSaveOpts, doc;

sourceFolder = Folder.selectDialog( 'Select the folder with Illustrator files you want to convert to png', '~' );

if ( sourceFolder != null )
{
  files = new Array();
	fileType = "*.ai";
	

	files = sourceFolder.getFiles( fileType );
	
	if ( files.length > 0 )
	{

		destFolder = Folder.selectDialog( 'Select the folder where you want to save the converted png files.', '~' );
		for ( i = 0; i < files.length; i++ )
		{
            sourceDoc = app.open(files[i]); // returns the document object
            doc = app.activeDocument;
            doc.layers.forEach((layer)=>{
                doc.layers[layer].visible=true;
            })
		}
		alert( 'Files are saved as EPS in ' + destFolder );
	}
	else
	{
		alert( 'No matching files found' );
	}
}