An example of discovering if a queryCommand works in a cross-browser, old-browser compliant way.

queryCommand.js

function runQueryCommand(command, showUI, value) {
  var commandWorked = false,
      initialDesignMode = document.designMode || "off";

  try {
    document.designMode = "on";
    commandWorked = !!(
        document.queryCommandSupported(command) &&
        document.queryCommandEnabled(command) &&
        document.queryCommand(command, showUI, value)
      );
  }
  catch (e) {
    // Do nothing
  }
  finally {
    document.designMode = initialDesignMode;
  }

  return commandWorked;
}



if (!runQueryCommand("copy", false, undefined)) {
  // Fallback to Flash-based alternative, e.g. ZeroClipboard
}

I haven't seen document.queryCommand() before - you meant document.execCommand in line 10?