# lowdb [](https://www.npmjs.org/package/lowdb) [](https://github.com/typicode/lowdb/actions/workflows/node.js.yml)
> Simple to use type-safe local JSON database 🦉
Read or create `db.json`
```js
const db = await JSONFilePreset('db.json', { posts: [] })
```
Update data using `Array.prototype.*` and automatically write to `db.json`
```js
const post = { id: 1, title: 'lowdb is awesome', views: 100 }
await db.update(({ posts }) => posts.push(post))
```
```js
// db.json
{
"posts": [
{ "id": 1, "title": "lowdb is awesome", "views": 100 }
]
}
```
In the same spirit, query using native `Array.prototype.*`
```js
const { posts } = db.data
const first = posts.at(0)
const results = posts.filter((post) => post.title.includes('lowdb'))
const post1 = posts.find((post) => post.id === 1)
const sortedPosts = posts.toSorted((a, b) => a.views - b.views)
```
It's that simple.
## Sponsors
[Become a sponsor and have your company logo here](https://github.com/sponsors/typicode) 👉 [GitHub Sponsors](https://github.com/sponsors/typicode)
## Features
- **Lightweight**
- **Minimalist**
- **TypeScript**
- **Plain JavaScript**
- Safe atomic writes
- Hackable:
- Change storage, file format (JSON, YAML, ...) or add encryption via [adapters](#adapters)
- Extend it with lodash, ramda, ... for super powers!
- Automatically switches to fast in-memory mode during tests
## Install
```sh
npm install lowdb
```
## Usage
_Lowdb is a pure ESM package. If you're having trouble using it in your project, please [read this](https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c)._
```js
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()
```
```js
// db.json
{
"posts": [ "hello world" ]
}
```
### TypeScript
You can use TypeScript to check your data types.
```ts
type Data = {
messages: string[]
}
const defaultData: Data = { messages: [] }
const db = await JSONPreset('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.
```ts
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 extends Low {
chain: lodash.ExpChain = lodash.chain(this).get('data')
}
const defaultData: Data = {
posts: [],
}
const adapter = new JSONFile('db.json', defaultData)
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/`](src/examples) directory.
## API
### Presets
Lowdb provides four presets for common cases.
- `JSONFilePreset(filename, defaultData)`
- `JSONFileSyncPreset(filename, defaultData)`
- `LocalStoragePreset(name, defaultData)`
- `SessionStoragePreset(name, defaultData)`
See [`src/examples/`](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)`
```js
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)`
```js
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.
```js
db.data // === null
db.read()
db.data // !== null
```
#### `db.write()`
Calls `adapter.write(db.data)`.
```js
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()`.
```js
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`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify).
For example:
```js
db.data = 'string'
db.data = [1, 2, 3]
db.data = { key: 'value' }
```
## Adapters
### Lowdb adapters
#### `JSONFile` `JSONFileSync`
Adapters for reading and writing JSON files.
```js
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/`](src/examples) directory.
```js
import { Memory, MemorySync } from 'lowdb'
new Low(new Memory(), {})
new LowSync(new MemorySync(), {})
```
#### `LocalStorage` `SessionStorage`
Synchronous adapter for `window.localStorage` and `window.sessionStorage`.
```js
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...).
```js
import { DataFile } from 'lowdb'
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:
```js
class AsyncAdapter {
read() {
/* ... */
} // should return Promise
write(data) {
/* ... */
} // should return Promise
}
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:
```js
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/`](src/adapters) for more examples.
#### Custom serialization
To create an adapter for another format than JSON, you can use `TextFile` or `TextFileSync`.
For example:
```js
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.