Lightweight Javascript DB for use in Node.js

NeDB seems to be what you are looking for. From the blurb:

Embedded persistent database for Node.js, written in Javascript, with no dependency (except npm modules of course). You can think of it as a SQLite for Node.js projects, which can be used with a simple require statement. The API is a subset of MongoDB's. You can use it as a persistent or an in-memory only datastore.

I had the same requirements as you but couldn't find a suitable database. nStore was promising but the API was not nearly complete enough and not very coherent.

That's why I made NeDB, which a dependency-less embedded database for Node.js projects. You can use it with a simple require(), it is persistent, and its API is the most commonly used subset of the very well-known MongoDB API.

https://github.com/louischatriot/nedb

Lokijs: A fast, in-memory document-oriented datastore for node.js, browser and cordova.

• In-memory Javascript Datastore wih Persistence
• In-Browser NoSQL db with syncing and persisting
• a Redis-style store an npm install away
• Persistable NoSQL db for Cordova
• Embeddable NoSQL db with Persistence for node-webkit

LokiJS to be the ideal solution:

• Mobile applications - especially HTML5 based (Cordova, Phonegap, etc.)
• Node.js embedded datastore for small-to-medium apps
• Embedded in desktop application with Node Webkit

https://github.com/techfort/LokiJS