When `data` in `lowdb` is updated `defaultData` (object) also updated.

Author: chokchaiCreated Dec 29, 2024Updated Jan 9, 2025

I don't know if this is expected behavior or not. But it feels off for me. It will cause issues when using defaultData multiple times. (ex. multiple lowdb instances using the same defaultData)

ISSUES:

defaultData is used as a reference for data in lowdb object.
when data in lowdb is updated the defaultData object content is also updated.
please check the example typescript code below.

typescript

import {JSONFilePreset} from "lowdb/node"

const defaultData = {
    name: "ALICE",
}

const db = await JSONFilePreset("test.json", defaultData)

// db.data should be ALICE
console.log(db.data.name === "ALICE") // true

// update db.data to BOB
db.data.name = "BOB"

// db.data now is BOB
console.log(db.data.name === "BOB") // true

// defaultData should still be ALICE
console.log(defaultData.name === "ALICE") // false

// defaultData becomes BOB instead (unexpected)
console.log(defaultData.name === "BOB") // true

SOLUTION:

deep clone object
using structuredClone fixed it.

typescript

const db = await JSONFilePreset("test.json", structuredClone(defaultData))

Source: typicode/lowdb

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