lowdb

lowdb

Simple to use local JSON database. Use native JavaScript API to query. Written in TypeScript. 🦉

// Edit db.json content using plain JavaScript
db.data.posts.push({ id: 1, title: 'lowdb is awesome' })

// Save to file
db.write()

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

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Sponsors

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Features

• Lightweight
• Minimalist
• TypeScript
• plain JavaScript
• Safe atomic writes
• Hackable:
  • Change storage, file format (JSON, YAML, ...) or add encryption via adapters
  • Add lodash, ramda, ... for super powers!

Install

npm install lowdb

Usage

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

Next.js: there's a known issue. Until it's fixed, please use this workaround or lowdb ^4.0.0.

// Remember to set type: module in package.json or use .mjs extension
import { join, dirname } from 'node:path'
import { fileURLToPath } from 'node:url'

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

// db.json file path
const __dirname = dirname(fileURLToPath(import.meta.url))
const file = join(__dirname, 'db.json')

// Configure lowdb to write data to JSON file
const adapter = new JSONFile(file)
const defaultData = { posts: [] }
const db = new Low(adapter, defaultData)

// Read data from JSON file, this will set db.data content
// If JSON file doesn't exist, defaultData is used instead
await db.read()

// Create and query items using plain JavaScript
db.data.posts.push('hello world')
const firstPost = db.data.posts[0]

// If you don't want to type db.data everytime, you can use destructuring assignment
const { posts } = db.data
posts.push('hello world')

// Finally write db.data content to file
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 adapter = new JSONFile<Data>('db.json')
const db = new Low<Data>(adapter)

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

Lodash

You can also add lodash or other utility libraries to improve lowdb.

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)
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

Classes

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

new Low(adapter)

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)

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 {}

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), {})

TextFile TextFileSync

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

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 { 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.