flat file database system especially for javascript

Is there any flat file database system that works well with javascript such as one in JSON format or similar. I have heard about mongodb, couchdb and others but it seems whole setup of it must be installed on the computer. Of course, I can't use `sqlite` beause I think I won't be able manipulate it via javascript and I don't want to use any server-side language for my small apps. I searched on google as well. Basically I am just looking for flat file database system that I can put in javascript app's folder (meaning portable database and app) and be able to use it anywhere, for example on some other computer without having to install any dependencies ? Does such portable flat file database exist out there ?