a base "class" for implementing a leveldown compatible API
Latest commit a714a5f Sep 12, 2017 @ralphtheninja ralphtheninja 2.7.0


Abstract LevelDOWN

LevelDB Logo

Build Status dependencies Greenkeeper badge


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 (options, callback) {
  // 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 (key, value, options, callback) {
  key = '_' + key // safety, to avoid key='__proto__'-type skullduggery 
  this._store[key] = value

FakeLevelDOWN.prototype._get = function (key, options, callback) {
  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 (key, options, callback) {
  delete this._store['_' + key]

// now use it in LevelUP

var levelup = require('levelup')

var db = levelup('/who/cares/', {
  // the 'db' option replaces LevelDOWN
  db: function (location) { return new FakeLevelDOWN(location) }

db.put('foo', 'bar', function (err) {
  if (err) throw err
  db.get('foo', function (err, value) {
    if (err) throw err
    console.log('Got foo =', value)

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

Extensible API

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.



An AbstractLevelDOWN based database can be in one of the following states:

  • 'new' - newly created, not opened or closed
  • 'opening' - waiting for the database to be opened
  • 'open' - successfully opened the database, available for use
  • 'closing' - waiting for the database to be closed
  • 'closed' - database has been successfully closed, should not be used

AbstractLevelDOWN#_open(options, callback)


AbstractLevelDOWN#_get(key, options, callback)

AbstractLevelDOWN#_put(key, value, options, callback)

AbstractLevelDOWN#_del(key, options, callback)

AbstractLevelDOWN#_batch(array, options, callback)

If batch() is called without arguments 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 a batch() operation without arguments 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.

AbstractLevelDOWN#_approximateSize(start, end, callback)




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 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.

AbstractChainedBatch#_put(key, value)



AbstractChainedBatch#_write(options, callback)




Returns true if db has the same public api as AbstractLevelDOWN, otherwise false. This is a utility function and it's not part of the extensible api.


AbstractLevelDOWN 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 contribution guide for more details.

License & Copyright

Copyright © 2013-2017 AbstractLevelDOWN contributors.

AbstractLevelDOWN 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.