JavaScript实现文件下载

function downloadFile(url){  var file =     Components.classes["@mozilla.org/file/local;1"]       .createInstance(Components.interfaces.nsILocalFile);   file.initWithPath("C:\\example.txt");  var ios = Components.classes['@mozilla.org/network/io-service;1']              .getService(Components.interfaces.nsIIOService);  var uri = ios.newURI(url, null, null);  var persist =     Components.classes["@mozilla.org/embedding/browser/nsWebBrowserPersist;1"]      .createInstance(Components.interfaces.nsIWebBrowserPersist);  const nsIWBP = Components.interfaces.nsIWebBrowserPersist;  const flags = nsIWBP.PERSIST_FLAGS_REPLACE_EXISTING_FILES;  persist.persistFlags = flags | nsIWBP.PERSIST_FLAGS_FROM_CACHE;  persist.saveURI(uri, null, null, null, null, file);}