作為輕量級的本地存儲方式，對于構建不依賴服務器的小型項目，用LowDB存儲和管理數(shù)據(jù)是十分理想的選擇。在Nodejs, Electron and browser等一些小型項目中經(jīng)常能看到LowDB的身影。
https://github.com/typicode/lowdb
1. 使用方式
npm install lowdb
或者：
yarn add lowdb
const low = require('lowdb');
const FileSync = require('lowdb/adapters/FileSync'); // 有多種適配器可選擇
const adapter = new FileSync('db.json'); // 申明一個適配器
const db = low(adapter);
db.defaults({posts: [], user: {}, count: 0})
.write();
db.get('posts')
.push({id: 1, title: 'lowdb is awesome'})
.write()
db.set('user.name', 'typicode')
.write()
db.update('count', n => n + 1)
.write()
運行程序會在項目中添加db.json文件，里面存儲了添加的數(shù)據(jù)：
{
"posts": [
{
"id": 1,
"title": "lowdb is awesome"
}
],
"user": {
"name": "typicode"
},
"count": 1
}
lowdb是基于lodash構建的，所以可以使用任何lodash強大的函數(shù)，比如: _.get() 和 _.find()，并且可以串聯(lián)地使用：
db.get('users')
.find({sex: 'male'})
.value()
2. API
函數(shù) 功能
low(adapter) 返回一個具有特定屬性和功能的 lodash chain
db.[...].write() / .value() 寫 / 讀數(shù)據(jù)
db.getState() / .setState() 獲取 / 設置數(shù)據(jù)庫的狀態(tài)
db._ 數(shù)據(jù)庫lodash的實例，可以利用這個添加自己的函數(shù)或者第三方的mixins,比如lodash-id
db._.mixin({
second: function(array) {
return array[1]
}
})
db.get('posts')
.second()
.value()
3. Adapters API
針對lowdb自帶的適配器：FileSync、FileAsync 和 LocalBrowser，有以下可選參數(shù)：
defaultValue: 文件不存在時的默認值；
serialize/deserialize: 寫之前和讀之后的操作。
const adapter = new FilSync('db.json',{
serialize: (data) => encrypt(JSON.stringify(data)),
deserialize: (data) => JSON.parse(decrypt(data))
})
4. 查詢
可以直接使用lodash的函數(shù)進行查詢。需要注意的是有些操作可能會導致原數(shù)據(jù)被修改，為了避免這種誤操作，需要使用 .cloneDeep()，操作都是惰性的，只有調用 .value()或 .write()后才會正式執(zhí)行。
檢查users是是否存在
db.has('users')
.value()
設置users
db.set('users', [])
.write()
排序、選擇
db.get('users')
.filter({sex: 'male'})
.sortBy('age')
.take(5)
.value()
獲取特定字段
db.get('users')
.map('name')
.value()
獲取數(shù)量
db.get('users')
.size()
.value()
獲取特定信息
db.get('users[0].name')
.value()
更新信息
db.get('users')
.find({name: 'Tom'})
.assign({name: 'Tim'})
.write()
刪除信息
db.get('users')
.remove({name: 'Time'})
.write()
移除屬性
db.unset('users.name)
.write()
深拷貝
db.get('users')
.cloneDeep()
.value()
5. 使用id索引
可以使用 shortid 和 lodash-id 為數(shù)據(jù)庫中的每一條記錄創(chuàng)建唯一的id索引，然后通過id檢索操作記錄：
const shortid = require('shortid')
const postId = db
.get('posts')
.push({ id: shortid.generate(), title: 'low!' })
.write()
.id
const post = db
.get('posts')
.find({ id: postId })
.value()
const lodashId = require('lodash-id')
const FileSync = require('lowdb/adapters/FileSync')
const adapter = new FileSync('db.json')
const db = low(adapter)
db._.mixin(lodashId)
// We need to set some default values, if the collection does not exist yet
// We also can store our collection
const collection = db
.defaults({ posts: [] })
.get('posts')
// Insert a new post...
const newPost = collection
.insert({ title: 'low!' })
.write()
// ...and retrieve it using its id
const post = collection
.getById(newPost.id)
.value()
6. 自定義Adapter
low( ) 函數(shù)接受自定義的Adapter
class MyStorage {
constructor() {
// ...
}
read() {
// Should return data (object or array) or a Promise
}
write(data) {
// Should return nothing or a Promise
}
}
const adapter = new MyStorage(args)
const db = low(adapter);
==============================================
英文官網(wǎng)介紹，更加簡潔
Install
npm install lowdb
Usage
Lowdb 3 is a pure ESM package. If you're having trouble importing it in your project, please read this.
import { join, dirname } from 'path'
import { Low, JSONFile } from 'lowdb'
import { fileURLToPath } from 'url'
const __dirname = dirname(fileURLToPath(import.meta.url));
// Use JSON file for storage
const file = join(__dirname, 'db.json')
const adapter = new JSONFile(file)
const db = new Low(adapter)
// Read data from JSON file, this will set db.data content
await db.read()
// If file.json doesn't exist, db.data will be null
// Set default data
// db.data = db.data || { posts: [] } // Node < v15.x
db.data ||= { posts: [] } // Node >= 15.x
// Create and query items using plain JS
db.data.posts.push('hello world')
const firstPost = db.data.posts[0]
// Alternatively, you can also use this syntax if you prefer
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 type check your data.
type Data = {
words: string[]
}
const adapter = new JSONFile<Data>('db.json')
const db = new Low(adapter)
db.data
.words
.push('foo') // ?
db.data
.words
.push(1) // ?
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 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
More examples
For CLI, server and browser usage, see examples/ directory.
API
Classes
Lowdb has two classes (for asynchronous and synchronous adapters).
new Low(adapter)
import { Low, JSONFile } from 'lowdb'
const db = new Low(new JSONFile('file.json'))
await db.read()
await db.write()
new LowSync(adapterSync)
import { LowSync, JSONFileSync } from 'lowdb'
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
JSONFileJSONFileSync
Adapters for reading and writing JSON files.
new Low(new JSONFile(filename))
new LowSync(new JSONFileSync(filename))
MemoryMemorySync
In-memory adapters. Useful for speeding up unit tests.
new Low(new Memory())
new LowSync(new MemorySync())
LocalStorage
Synchronous adapter for window.localStorage.
new LowSync(new LocalStorage(name))
TextFileTextFileSync
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, TextFile } from 'lowdb'
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.
