reselect/website/docs/usage/handling-empty-array-results.mdx

---
id: handling-empty-array-results
title: Handling Empty Array Results
sidebar_label: Handling Empty Array Results
hide_title: true
description: Handling Empty Array Results
---

import Tabs from '@theme/Tabs'
import TabItem from '@theme/TabItem'
import { InternalLinks } from '@site/src/components/InternalLinks'

# Handling Empty Array Results

To reduce recalculations, use a predefined empty array when `array.filter` or similar methods result in an empty array.

So you can have a pattern like this:

{/* START: handling-empty-array-results/firstPattern.ts */}

<Tabs
  groupId='language'
  defaultValue='ts'
  values={[
    {label: 'TypeScript', value: 'ts'},
    {label: 'JavaScript', value: 'js'},
  ]}>
  <TabItem value='ts'>

```ts title="handling-empty-array-results/firstPattern.ts"
import { createSelector } from 'reselect'

export interface RootState {
  todos: {
    id: number
    title: string
    description: string
    completed: boolean
  }[]
}

const EMPTY_ARRAY: [] = []

const selectCompletedTodos = createSelector(
  [(state: RootState) => state.todos],
  todos => {
    const completedTodos = todos.filter(todo => todo.completed === true)
    return completedTodos.length === 0 ? EMPTY_ARRAY : completedTodos
  }
)
```

  </TabItem>
  <TabItem value='js'>

```js title="handling-empty-array-results/firstPattern.js"
import { createSelector } from 'reselect'

const EMPTY_ARRAY = []

const selectCompletedTodos = createSelector([state => state.todos], todos => {
  const completedTodos = todos.filter(todo => todo.completed === true)
  return completedTodos.length === 0 ? EMPTY_ARRAY : completedTodos
})
```

  </TabItem>
</Tabs>

{/* END: handling-empty-array-results/firstPattern.ts */}

Or to avoid repetition, you can create a wrapper function and reuse it:

{/* START: handling-empty-array-results/fallbackToEmptyArray.ts */}

<Tabs
  groupId='language'
  defaultValue='ts'
  values={[
    {label: 'TypeScript', value: 'ts'},
    {label: 'JavaScript', value: 'js'},
  ]}>
  <TabItem value='ts'>

```ts title="handling-empty-array-results/fallbackToEmptyArray.ts"
import { createSelector } from 'reselect'
import type { RootState } from './firstPattern'

const EMPTY_ARRAY: [] = []

export const fallbackToEmptyArray = <T>(array: T[]) => {
  return array.length === 0 ? EMPTY_ARRAY : array
}

const selectCompletedTodos = createSelector(
  [(state: RootState) => state.todos],
  todos => {
    return fallbackToEmptyArray(todos.filter(todo => todo.completed === true))
  }
)
```

  </TabItem>
  <TabItem value='js'>

```js title="handling-empty-array-results/fallbackToEmptyArray.js"
import { createSelector } from 'reselect'

const EMPTY_ARRAY = []

export const fallbackToEmptyArray = array => {
  return array.length === 0 ? EMPTY_ARRAY : array
}

const selectCompletedTodos = createSelector([state => state.todos], todos => {
  return fallbackToEmptyArray(todos.filter(todo => todo.completed === true))
})
```

  </TabItem>
</Tabs>

{/* END: handling-empty-array-results/fallbackToEmptyArray.ts */}

This way if the <InternalLinks.ResultFunction /> returns an empty array twice in a row, your component will not re-render due to a stable empty array reference:

```ts
const completedTodos = selectCompletedTodos(store.getState())

store.dispatch(addTodo())

console.log(completedTodos === selectCompletedTodos(store.getState())) //=> true
```