---
url: /reference/core/defaultdataadapter.md
---

# DefaultDataAdapter

```ts
import { DefaultDataAdapter } from '@signaldb/core'
```

Keeps every collection in memory on the main thread and answers queries from
there. Writes are applied immediately and persisted in the background, which is
what makes an [optimistic UI](/optimistic-ui/) fall out for free.

This is the adapter to use unless you have a reason not to, and the one a
collection constructed without an adapter uses.

## Usage

```js
import { Collection, DefaultDataAdapter } from '@signaldb/core'
import createLocalStorageAdapter from '@signaldb/localstorage'

const dataAdapter = new DefaultDataAdapter({
  storage: name => createLocalStorageAdapter(name),
})

const Posts = new Collection('posts', dataAdapter)
const Authors = new Collection('authors', dataAdapter)
```

Without a `storage` function the data lives only in memory and is gone on
reload — useful for tests and for data you derive rather than keep.

## Options

```ts
new DefaultDataAdapter(options?: {
  storage?: (name: string) => StorageAdapter<any, any> | undefined
  onError?: (name: string, error: Error) => void
})
```

* `storage`: Called once per collection with the collection's name, and returns the [storage adapter](/data-persistence/) that collection persists to. Return `undefined` to keep a particular collection in memory only.
* `onError`: Called when setting up a collection's storage adapter or loading its stored items fails; without it, the error is logged with `console.error`. The collection's `ready()` then rejects with the same error and `isReady()` stays `false`, while the collection keeps working in memory. A write whose storage operation fails rejects its own promise instead.

## Queries are synchronous

Because the data is in memory, `find().fetch()` returns the result directly and
`Cursor#isLoading()` is never `true` after the collection is ready. Waiting for
[`collection.ready()`](/reference/core/collection/#ready) is still necessary
before the first read: until the storage adapter has loaded, the collection is
empty, and an empty result is indistinguishable from a real one.

## Indices

The `indices` a collection declares are used here. A selector no index covers is
answered by scanning the collection in memory — fine while it is small, and the
thing to fix when it grows. See
[what this means for your collections](/data-persistence/#what-this-means-for-your-collections).