An abstract prototype matching the LevelDOWN API

Abstract LevelDOWN

An abstract prototype matching the LevelDOWN API. Useful for extending LevelUP functionality by providing a replacement to LevelDOWN.

As of version 0.7, LevelUP allows you to pass a 'db' option when you create a new instance. This will override the default LevelDOWN store with a LevelDOWN API compatible object.

Abstract LevelDOWN provides a simple, operational noop base prototype that's ready for extending. By default, all operations have sensible "noops" (operations that essentially do nothing). For example, simple operations such as .open(callback) and .close(callback) will simply invoke the callback (on a next tick). More complex operations perform sensible actions, for example: .get(key, callback) will always return a 'NotFound' Error on the callback.

You add functionality by implementing the underscore versions of the operations. For example, to implement a put() operation you add a _put() method to your object. Each of these underscore methods override the default noop operations and are always provided with consistent arguments, regardless of what is passed in by the client.

Additionally, all methods provide argument checking and sensible defaults for optional arguments. All bad-argument errors are compatible with LevelDOWN (they pass the LevelDOWN method arguments tests). For example, if you call .open() without a callback argument you'll get an Error('open() requires a callback argument'). Where optional arguments are involved, your underscore methods will receive sensible defaults. A .get(key, callback) will pass through to a ._get(key, options, callback) where the options argument is an empty object.

A simplistic in-memory LevelDOWN replacement

var util = require('util')
  , AbstractLevelDOWN = require('./').AbstractLevelDOWN
// constructor, passes through the 'location' argument to the AbstractLevelDOWN constructor 
function FakeLevelDOWN (location) {
  AbstractLevelDOWN.call(this, location)
// our new prototype inherits from AbstractLevelDOWN 
util.inherits(FakeLevelDOWN, AbstractLevelDOWN)
// implement some methods 
FakeLevelDOWN.prototype._open = function (optionscallback) {
  // initialise a memory storage object 
  this._store = {}
  // optional use of nextTick to be a nice async citizen 
  process.nextTick(function () { callback(null, this) }.bind(this))
FakeLevelDOWN.prototype._put = function (keyvalueoptionscallback) {
  key = '_' + key // safety, to avoid key='__proto__'-type skullduggery  
  this._store[key] = value
FakeLevelDOWN.prototype._get = function (keyoptionscallback) {
  var value = this._store['_' + key]
  if (value === undefined) {
    // 'NotFound' error, consistent with LevelDOWN API 
    return process.nextTick(function () { callback(new Error('NotFound')) })
  process.nextTick(function () {
    callback(null, value)
FakeLevelDOWN.prototype._del = function (keyoptionscallback) {
  delete this._store['_' + key]
// now use it in LevelUP 
var levelup = require('levelup')
var db = levelup('/who/cares/', {
  // the 'db' option replaces LevelDOWN 
  dbfunction (location) { return new FakeLevelDOWN(location) }
db.put('foo', 'bar', function (err) {
  if (err) throw err
  db.get('foo', function (errvalue) {
    if (err) throw err
    console.log('Got foo =', value)

See MemDOWN if you are looking for a complete in-memory replacement for LevelDOWN.

Remember that each of these methods, if you implement them, will receive exactly the number and order of arguments described. Optional arguments will be converted to sensible defaults.

If batch() is called without argument or with only an options object then it should return a Batch object with chainable methods. Otherwise it will invoke a classic batch operation.

By default an batch() operation without argument returns a blank AbstractChainedBatch object. The prototype is available on the main exports for you to extend. If you want to implement chainable batch operations then you should extend the AbstractChaindBatch and return your object in the _chainedBatch() method.

By default an iterator() operation returns a blank AbstractIterator object. The prototype is available on the main exports for you to extend. If you want to implement iterator operations then you should extend the AbstractIterator and return your object in the _iterator(options) method.

AbstractIterator implements the basic state management found in LevelDOWN. It keeps track of when a next() is in progress and when an end() has been called so it doesn't allow concurrent next() calls, it does it allow end() while a next() is in progress and it doesn't allow either next() or end() after end() has been called.

Provided with the current instance of AbstractLevelDOWN by default.

Provided with the current instance of AbstractLevelDOWN by default.


Abstract LevelDOWN is an OPEN Open Source Project. This means that:

Individuals making significant and valuable contributions are given commit-access to the project to contribute as they see fit. This project is more like an open wiki than a standard guarded open source project.

See the CONTRIBUTING.md file for more details.

Abstract LevelDOWN is only possible due to the excellent work of the following contributors:

Rod VaggGitHub/rvaggTwitter/@rvagg
John ChesleyGitHub/cheslesTwitter/@chesles
Jake VerbatenGitHub/raynosTwitter/@raynos2
Dominic TarrGitHub/dominictarrTwitter/@dominictarr
Max OgdenGitHub/maxogdenTwitter/@maxogden
Lars-Magnus SkogGitHub/ralphtheninjaTwitter/@ralphtheninja
David BjörklundGitHub/keslaTwitter/@david_bjorklund
Julian GruberGitHub/juliangruberTwitter/@juliangruber
Paolo FragomeniGitHub/hij1nxTwitter/@hij1nx
Anton WhalleyGitHub/No9Twitter/@antonwhalley
Matteo CollinaGitHub/mcollinaTwitter/@matteocollina
Pedro TeixeiraGitHub/pgteTwitter/@pgte
James HallidayGitHub/substackTwitter/@substack
Thomas Watson SteenGitHub/watsonTwitter/@wa7son

License & copyright

Copyright (c) 2012-2014 Abstract LevelDOWN contributors (listed above).

Abstract LevelDOWN is licensed under the MIT license. All rights not explicitly granted in the MIT license are reserved. See the included LICENSE.md file for more details.