lowdb README

The project's README from github.com/typicode/lowdb. As one markdown file: README.md.

lowdb Node.js CI

Simple to use type-safe local JSON database 🦉

If you know JavaScript, you know how to use lowdb.

Read or create db.json

const db = await JSONFilePreset('db.json', { posts: [] })

Use plain JavaScript to change data

const post = { id: 1, title: 'lowdb is awesome', views: 100 }

// In two steps
db.data.posts.push(post)
await db.write()

// Or in one
await db.update(({ posts }) => posts.push(post))

// db.json
{
  "posts": [
    { "id": 1, "title": "lowdb is awesome", "views": 100 }
  ]
}

In the same spirit, query using native Array functions:

const { posts } = db.data

posts.at(0) // First post
posts.filter((post) => post.title.includes('lowdb')) // Filter by title
posts.find((post) => post.id === 1) // Find by id
posts.toSorted((a, b) => a.views - b.views) // Sort by views

It's that simple. db.data is just a JavaScript object, no magic.

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Features

Install

npm install lowdb

Usage

Lowdb is a pure ESM package. If you're having trouble using it in your project, please read this.

import { JSONFilePreset } from 'lowdb/node'

// Read or create db.json
const defaultData = { posts: [] }
const db = await JSONFilePreset('db.json', defaultData)

// Update db.json
await db.update(({ posts }) => posts.push('hello world'))

// Alternatively you can call db.write() explicitely later
// to write to db.json
db.data.posts.push('hello world')
await db.write()

// db.json
{
  "posts": [ "hello world" ]
}

TypeScript

You can use TypeScript to check your data types.

type Data = {
  messages: string[]
}

const defaultData: Data = { messages: [] }
const db = await JSONPreset<Data>('db.json', defaultData)

db.data.messages.push('foo') // ✅ Success
db.data.messages.push(1) // ❌ TypeScript error

Lodash

You can extend lowdb with Lodash (or other libraries). To be able to extend it, we're not using JSONPreset here. Instead, we're using lower components.

import { Low } from 'lowdb'
import { JSONFile } from 'lowdb/node'
import lodash from 'lodash'

type Post = {
  id: number
  title: string
}

type Data = {
  posts: Post[]
}

// Extend Low class with a new `chain` field
class LowWithLodash<T> extends Low<T> {
  chain: lodash.ExpChain<this['data']> = lodash.chain(this).get('data')
}

const defaultData: Data = {
  posts: [],
}
const adapter = new JSONFile<Data>('db.json')

const db = new LowWithLodash(adapter, defaultData)
await db.read()

// Instead of db.data use db.chain to access lodash API
const post = db.chain.get('posts').find({ id: 1 }).value() // Important: value() must be called to execute chain

CLI, Server, Browser and in tests usage

See src/examples/ directory.

API

Presets

Lowdb provides four presets for common cases.

See src/examples/ directory for usage.

Lowdb is extremely flexible, if you need to extend it or modify its behavior, use the classes and adapters below instead of the presets.

Classes

Lowdb has two classes (for asynchronous and synchronous adapters).

new Low(adapter, defaultData)

import { Low } from 'lowdb'
import { JSONFile } from 'lowdb/node'

const db = new Low(new JSONFile('file.json'), {})
await db.read()
await db.write()

new LowSync(adapterSync, defaultData)

import { LowSync } from 'lowdb'
import { JSONFileSync } from 'lowdb/node'

const db = new LowSync(new JSONFileSync('file.json'), {})
db.read()
db.write()

Methods

db.read()

Calls adapter.read() and sets db.data.

Note: JSONFile and JSONFileSync adapters will set db.data to null if file doesn't exist.

db.data // === null
db.read()
db.data // !== null

db.write()

Calls adapter.write(db.data).

db.data = { posts: [] }
db.write() // file.json will be { posts: [] }
db.data = {}
db.write() // file.json will be {}

db.update(fn)

Calls fn() then db.write().

db.update((data) => {
  // make changes to data
  // ...
})
// files.json will be updated

Properties

db.data

Holds your db content. If you're using the adapters coming with lowdb, it can be any type supported by JSON.stringify.

For example:

db.data = 'string'
db.data = [1, 2, 3]
db.data = { key: 'value' }

Adapters

Lowdb adapters

JSONFile JSONFileSync

Adapters for reading and writing JSON files.

import { JSONFile, JSONFileSync } from 'lowdb/node'

new Low(new JSONFile(filename), {})
new LowSync(new JSONFileSync(filename), {})

Memory MemorySync

In-memory adapters. Useful for speeding up unit tests. See src/examples/ directory.

import { Memory, MemorySync } from 'lowdb'

new Low(new Memory(), {})
new LowSync(new MemorySync(), {})

LocalStorage SessionStorage

Synchronous adapter for window.localStorage and window.sessionStorage.

import { LocalStorage, SessionStorage } from 'lowdb/browser'
new LowSync(new LocalStorage(name), {})
new LowSync(new SessionStorage(name), {})

Utility adapters

TextFile TextFileSync

Adapters for reading and writing text. Useful for creating custom adapters.

DataFile DataFileSync

Adapters for easily supporting other data formats or adding behaviors (encrypt, compress...).

import { DataFile } from 'lowdb/node'
new DataFile(filename, {
  parse: YAML.parse,
  stringify: YAML.stringify
})
new DataFile(filename, {
  parse: (data) => { decypt(JSON.parse(data)) },
  stringify: (str) => { encrypt(JSON.stringify(str)) }
})

Third-party adapters

If you've published an adapter for lowdb, feel free to create a PR to add it here.

Writing your own adapter

You may want to create an adapter to write db.data to YAML, XML, encrypt data, a remote storage, ...

An adapter is a simple class that just needs to expose two methods:

class AsyncAdapter {
  read() {
    /* ... */
  } // should return Promise<data>
  write(data) {
    /* ... */
  } // should return Promise<void>
}

class SyncAdapter {
  read() {
    /* ... */
  } // should return data
  write(data) {
    /* ... */
  } // should return nothing
}

For example, let's say you have some async storage and want to create an adapter for it:

import { Low } from 'lowdb'
import { api } from './AsyncStorage'

class CustomAsyncAdapter {
  // Optional: your adapter can take arguments
  constructor(args) {
    // ...
  }

  async read() {
    const data = await api.read()
    return data
  }

  async write(data) {
    await api.write(data)
  }
}

const adapter = new CustomAsyncAdapter()
const db = new Low(adapter, {})

See src/adapters/ for more examples.

Custom serialization

To create an adapter for another format than JSON, you can use TextFile or TextFileSync.

For example:

import { Adapter, Low } from 'lowdb'
import { TextFile } from 'lowdb/node'
import YAML from 'yaml'

class YAMLFile {
  constructor(filename) {
    this.adapter = new TextFile(filename)
  }

  async read() {
    const data = await this.adapter.read()
    if (data === null) {
      return null
    } else {
      return YAML.parse(data)
    }
  }

  write(obj) {
    return this.adapter.write(YAML.stringify(obj))
  }
}

const adapter = new YAMLFile('file.yaml')
const db = new Low(adapter, {})

Limits

Lowdb doesn't support Node's cluster module.

If you have large JavaScript objects (~10-100MB) you may hit some performance issues. This is because whenever you call db.write, the whole db.data is serialized using JSON.stringify and written to storage.

Depending on your use case, this can be fine or not. It can be mitigated by doing batch operations and calling db.write only when you need it.

If you plan to scale, it's highly recommended to use databases like PostgreSQL or MongoDB instead.

lowdb (typicode/lowdb)

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22.6K ·GitHub
Lowdb
What is this?lowdb is a small, type-safe local JSON database for JavaScript: you read or create a db.json file and change data with plain JavaScript, including native Array functions.
So it's a library I install, not a database server I have to run?It's a library you install from npm, not a server. There's no process to run — db.data is just a JavaScript object that lowdb keeps in sync with a JSON file.
How do I start using it in a project?Run npm install lowdb, then create the database with JSONFilePreset('db.json', defaultData). After that db.update(fn) changes data and writes for you, or use db.read() and db.write() manually.
Does it need anything besides npm?Just Node.js 18 or newer — it's a pure ESM package with a single runtime dependency (steno), no database binary or background service.
Any limits I should know about before betting on it?It doesn't support Node's cluster module, and large objects (~10-100MB) get slow because every db.write serializes the whole database with JSON.stringify. If you plan to scale, the README recommends PostgreSQL or MongoDB instead.
Can I use it in the browser too?Yes. Besides Node.js it ships LocalStorage and SessionStorage presets for window.localStorage and window.sessionStorage, and the README points to CLI, Server and Browser examples.
Where does my data actually live?In a plain JSON file on disk (db.json by default) that you control. Nothing runs in the cloud — no server, accounts or telemetry.
Can I use it from TypeScript?Yes — lowdb is type-safe, so you can describe db.data with TypeScript types and catch shape errors at compile time.
Can it save data in other formats, like YAML?Yes, via adapters: lowdb ships TextFile and DataFile utility adapters, and a custom adapter is just a class with two methods, so it can target YAML, XML, encryption, compression or remote storage.
Can I use Lodash or Ramda with it?Yes. Instead of the JSONPreset shortcut you use the lower-level classes, then extend your queries with Lodash or Ramda as the README suggests.
What happens when tests run against it?It automatically switches to a fast in-memory mode during tests, and the Memory and MemorySync adapters exist for the same purpose.
What are similar projects?nedb, rxdb, Dexie.js, store.js, localForage, redux-persist, ultrastorage, gun
Which topics is it in?lodash, in-memory, localstorage, embedded-database, nosql, storage, lightweight, json, electron, beginner-friendly, browser, local-first
Creator
typicode
Size
1.1 MB
Stars
22.6K
Updated
CommitMar 27Releasev7.0.0Dec 26
Age
12y
Commits · 90 Days