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---
id: weakMapMemoize
title: weakMapMemoize
sidebar_label: weakMapMemoize
hide_title: true
description: 'weakMapMemoize'
---

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

# `weakMapMemoize`

<InternalLinks.LruMemoize /> has to be explicitly configured to have a cache
size larger than 1, and uses an LRU cache internally.

`weakMapMemoize` creates a tree of <ExternalLinks.WeakMap />-based cache nodes based on the identity of the arguments it's been called with (in this case, the extracted values from your input selectors). **This allows `weakMapMemoize` to have an effectively infinite cache size**. Cache results will be kept in memory as long as references to the arguments still exist, and then cleared out as the arguments are garbage-collected.

## Design Tradeoffs

- Pros:

  - It has an effectively infinite cache size, but you have no control over
    how long values are kept in cache as it's based on garbage collection and <ExternalLinks.WeakMap />s.

- Cons:
  - There's currently no way to alter the argument comparisons. They're based on <ExternalLinks.ReferenceEqualityCheck text='strict reference equality' />.

## Use Cases

- This memoizer is likely best used for cases where you need to call the
  same selector instance with many different arguments, such as a single
  selector instance that is used in a list item component and called with
  item IDs like:

```ts
useSelector(state => selectSomeData(state, id))
```

Prior to `weakMapMemoize`, you had this problem:

{/* START: weakMapMemoize/cacheSizeProblem.ts */}

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

```ts title="weakMapMemoize/cacheSizeProblem.ts"
import { createSelector } from 'reselect'

export interface RootState {
  items: { id: number; category: string; name: string }[]
}

const state: RootState = {
  items: [
    { id: 1, category: 'Electronics', name: 'Wireless Headphones' },
    { id: 2, category: 'Books', name: 'The Great Gatsby' },
    { id: 3, category: 'Home Appliances', name: 'Blender' },
    { id: 4, category: 'Stationery', name: 'Sticky Notes' }
  ]
}

const selectItemsByCategory = createSelector(
  [
    (state: RootState) => state.items,
    (state: RootState, category: string) => category
  ],
  (items, category) => items.filter(item => item.category === category)
)

selectItemsByCategory(state, 'Electronics') // Selector runs
selectItemsByCategory(state, 'Electronics')
selectItemsByCategory(state, 'Stationery') // Selector runs
selectItemsByCategory(state, 'Electronics') // Selector runs again!
```

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

```js title="weakMapMemoize/cacheSizeProblem.js"
import { createSelector } from 'reselect'

const state = {
  items: [
    { id: 1, category: 'Electronics', name: 'Wireless Headphones' },
    { id: 2, category: 'Books', name: 'The Great Gatsby' },
    { id: 3, category: 'Home Appliances', name: 'Blender' },
    { id: 4, category: 'Stationery', name: 'Sticky Notes' }
  ]
}

const selectItemsByCategory = createSelector(
  [state => state.items, (state, category) => category],
  (items, category) => items.filter(item => item.category === category)
)

selectItemsByCategory(state, 'Electronics') // Selector runs
selectItemsByCategory(state, 'Electronics')
selectItemsByCategory(state, 'Stationery') // Selector runs
selectItemsByCategory(state, 'Electronics') // Selector runs again!
```

  </TabItem>
</Tabs>

{/* END: weakMapMemoize/cacheSizeProblem.ts */}

Before you could solve this in a number of different ways:

1. Set the `maxSize` with <InternalLinks.LruMemoize />:

{/* START: weakMapMemoize/setMaxSize.ts */}

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

```ts title="weakMapMemoize/setMaxSize.ts"
import { createSelector, lruMemoize } from 'reselect'
import type { RootState } from './cacheSizeProblem'

const selectItemsByCategory = createSelector(
  [
    (state: RootState) => state.items,
    (state: RootState, category: string) => category
  ],
  (items, category) => items.filter(item => item.category === category),
  {
    memoize: lruMemoize,
    memoizeOptions: {
      maxSize: 10
    }
  }
)
```

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

```js title="weakMapMemoize/setMaxSize.js"
import { createSelector, lruMemoize } from 'reselect'

const selectItemsByCategory = createSelector(
  [state => state.items, (state, category) => category],
  (items, category) => items.filter(item => item.category === category),
  {
    memoize: lruMemoize,
    memoizeOptions: {
      maxSize: 10
    }
  }
)
```

  </TabItem>
</Tabs>

{/* END: weakMapMemoize/setMaxSize.ts */}

But this required having to know the cache size ahead of time.

2. Create unique selector instances using <ExternalLinks.UseMemo />.

{/* START: weakMapMemoize/withUseMemo.tsx */}

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

```tsx title="weakMapMemoize/withUseMemo.tsx"
import type { FC } from 'react'
import { useMemo } from 'react'
import { useSelector } from 'react-redux'
import { createSelector } from 'reselect'
import type { RootState } from './cacheSizeProblem'

const makeSelectItemsByCategory = (category: string) =>
  createSelector([(state: RootState) => state.items], items =>
    items.filter(item => item.category === category)
  )

interface Props {
  category: string
}

const MyComponent: FC<Props> = ({ category }) => {
  const selectItemsByCategory = useMemo(
    () => makeSelectItemsByCategory(category),
    [category]
  )

  const itemsByCategory = useSelector(selectItemsByCategory)

  return (
    <div>
      {itemsByCategory.map(item => (
        <div key={item.id}>{item.name}</div>
      ))}
    </div>
  )
}
```

  </TabItem>
  <TabItem value='jsx'>

```jsx title="weakMapMemoize/withUseMemo.jsx"
import { useMemo } from 'react'
import { useSelector } from 'react-redux'
import { createSelector } from 'reselect'

const makeSelectItemsByCategory = category =>
  createSelector([state => state.items], items =>
    items.filter(item => item.category === category)
  )

const MyComponent = ({ category }) => {
  const selectItemsByCategory = useMemo(
    () => makeSelectItemsByCategory(category),
    [category]
  )

  const itemsByCategory = useSelector(selectItemsByCategory)

  return (
    <div>
      {itemsByCategory.map(item => (
        <div key={item.id}>{item.name}</div>
      ))}
    </div>
  )
}
```

  </TabItem>
</Tabs>

{/* END: weakMapMemoize/withUseMemo.tsx */}

3. Use <ExternalLinks.ReReselect />:

```ts
import { createCachedSelector } from 're-reselect'

const selectItemsByCategory = createCachedSelector(
  [
    (state: RootState) => state.items,
    (state: RootState, category: string) => category
  ],
  (items, category) => items.filter(item => item.category === category)
)((state: RootState, category: string) => category)
```

Starting in 5.0.0, you can eliminate this problem using `weakMapMemoize`.

{/* START: weakMapMemoize/cacheSizeSolution.ts */}

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

```ts title="weakMapMemoize/cacheSizeSolution.ts"
import { createSelector, weakMapMemoize } from 'reselect'
import type { RootState } from './cacheSizeProblem'

const state: RootState = {
  items: [
    { id: 1, category: 'Electronics', name: 'Wireless Headphones' },
    { id: 2, category: 'Books', name: 'The Great Gatsby' },
    { id: 3, category: 'Home Appliances', name: 'Blender' },
    { id: 4, category: 'Stationery', name: 'Sticky Notes' }
  ]
}

const selectItemsByCategory = createSelector(
  [
    (state: RootState) => state.items,
    (state: RootState, category: string) => category
  ],
  (items, category) => items.filter(item => item.category === category),
  {
    memoize: weakMapMemoize,
    argsMemoize: weakMapMemoize
  }
)

selectItemsByCategory(state, 'Electronics') // Selector runs
selectItemsByCategory(state, 'Electronics') // Cached
selectItemsByCategory(state, 'Stationery') // Selector runs
selectItemsByCategory(state, 'Electronics') // Still cached!
```

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

```js title="weakMapMemoize/cacheSizeSolution.js"
import { createSelector, weakMapMemoize } from 'reselect'

const state = {
  items: [
    { id: 1, category: 'Electronics', name: 'Wireless Headphones' },
    { id: 2, category: 'Books', name: 'The Great Gatsby' },
    { id: 3, category: 'Home Appliances', name: 'Blender' },
    { id: 4, category: 'Stationery', name: 'Sticky Notes' }
  ]
}

const selectItemsByCategory = createSelector(
  [state => state.items, (state, category) => category],
  (items, category) => items.filter(item => item.category === category),
  {
    memoize: weakMapMemoize,
    argsMemoize: weakMapMemoize
  }
)

selectItemsByCategory(state, 'Electronics') // Selector runs
selectItemsByCategory(state, 'Electronics') // Cached
selectItemsByCategory(state, 'Stationery') // Selector runs
selectItemsByCategory(state, 'Electronics') // Still cached!
```

  </TabItem>
</Tabs>

{/* END: weakMapMemoize/cacheSizeSolution.ts */}

This solves the problem of having to know and set the cache size prior to creating a memoized selector. Because `weakMapMemoize` essentially provides a dynamic cache size out of the box.

## Parameters

| Name   | Description                  |
| :----- | :--------------------------- |
| `func` | The function to be memoized. |

## Returns

A memoized function with a `.clearCache()` method attached.

## Type Parameters

| Name   | Description                                |
| :----- | :----------------------------------------- |
| `Func` | The type of the function that is memoized. |

## Examples

### Using `weakMapMemoize` with `createSelector`

{/* START: weakMapMemoize/usingWithCreateSelector.ts */}

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

```ts title="weakMapMemoize/usingWithCreateSelector.ts"
import { createSelector, weakMapMemoize } from 'reselect'
import type { RootState } from './cacheSizeProblem'

const state: RootState = {
  items: [
    { id: 1, category: 'Electronics', name: 'Wireless Headphones' },
    { id: 2, category: 'Books', name: 'The Great Gatsby' },
    { id: 3, category: 'Home Appliances', name: 'Blender' },
    { id: 4, category: 'Stationery', name: 'Sticky Notes' }
  ]
}

const selectItemsByCategory = createSelector(
  [
    (state: RootState) => state.items,
    (state: RootState, category: string) => category
  ],
  (items, category) => items.filter(item => item.category === category),
  {
    memoize: weakMapMemoize,
    argsMemoize: weakMapMemoize
  }
)

selectItemsByCategory(state, 'Electronics') // Selector runs
selectItemsByCategory(state, 'Electronics')
selectItemsByCategory(state, 'Stationery') // Selector runs
selectItemsByCategory(state, 'Electronics')
```

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

```js title="weakMapMemoize/usingWithCreateSelector.js"
import { createSelector, weakMapMemoize } from 'reselect'

const state = {
  items: [
    { id: 1, category: 'Electronics', name: 'Wireless Headphones' },
    { id: 2, category: 'Books', name: 'The Great Gatsby' },
    { id: 3, category: 'Home Appliances', name: 'Blender' },
    { id: 4, category: 'Stationery', name: 'Sticky Notes' }
  ]
}

const selectItemsByCategory = createSelector(
  [state => state.items, (state, category) => category],
  (items, category) => items.filter(item => item.category === category),
  {
    memoize: weakMapMemoize,
    argsMemoize: weakMapMemoize
  }
)

selectItemsByCategory(state, 'Electronics') // Selector runs
selectItemsByCategory(state, 'Electronics')
selectItemsByCategory(state, 'Stationery') // Selector runs
selectItemsByCategory(state, 'Electronics')
```

  </TabItem>
</Tabs>

{/* END: weakMapMemoize/usingWithCreateSelector.ts */}

### Using `weakMapMemoize` with `createSelectorCreator`

{/* START: weakMapMemoize/usingWithCreateSelectorCreator.ts */}

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

```ts title="weakMapMemoize/usingWithCreateSelectorCreator.ts"
import { createSelectorCreator, weakMapMemoize } from 'reselect'
import type { RootState } from './cacheSizeProblem'

const state: RootState = {
  items: [
    { id: 1, category: 'Electronics', name: 'Wireless Headphones' },
    { id: 2, category: 'Books', name: 'The Great Gatsby' },
    { id: 3, category: 'Home Appliances', name: 'Blender' },
    { id: 4, category: 'Stationery', name: 'Sticky Notes' }
  ]
}

const createSelectorWeakMap = createSelectorCreator({
  memoize: weakMapMemoize,
  argsMemoize: weakMapMemoize
})

const selectItemsByCategory = createSelectorWeakMap(
  [
    (state: RootState) => state.items,
    (state: RootState, category: string) => category
  ],
  (items, category) => items.filter(item => item.category === category)
)

selectItemsByCategory(state, 'Electronics') // Selector runs
selectItemsByCategory(state, 'Electronics')
selectItemsByCategory(state, 'Stationery') // Selector runs
selectItemsByCategory(state, 'Electronics')
```

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

```js title="weakMapMemoize/usingWithCreateSelectorCreator.js"
import { createSelectorCreator, weakMapMemoize } from 'reselect'

const state = {
  items: [
    { id: 1, category: 'Electronics', name: 'Wireless Headphones' },
    { id: 2, category: 'Books', name: 'The Great Gatsby' },
    { id: 3, category: 'Home Appliances', name: 'Blender' },
    { id: 4, category: 'Stationery', name: 'Sticky Notes' }
  ]
}

const createSelectorWeakMap = createSelectorCreator({
  memoize: weakMapMemoize,
  argsMemoize: weakMapMemoize
})

const selectItemsByCategory = createSelectorWeakMap(
  [state => state.items, (state, category) => category],
  (items, category) => items.filter(item => item.category === category)
)

selectItemsByCategory(state, 'Electronics') // Selector runs
selectItemsByCategory(state, 'Electronics')
selectItemsByCategory(state, 'Stationery') // Selector runs
selectItemsByCategory(state, 'Electronics')
```

  </TabItem>
</Tabs>

{/* END: weakMapMemoize/usingWithCreateSelectorCreator.ts */}