v5.3.0

This feature release adds a maxSize option to weakMapMemoize to bound cache growth, adds a new development-mode check that warns when a selector's cache grows without bound, improves memoization performance, removes the experimental unstable_autotrackMemoize, and updates our TypeScript support matrix and documentation.

Changelog

maxSize Option for weakMapMemoize

weakMapMemoize has been the default memoizer since v5.0, and its main benefit is the infinite cache size. Prior to v5, selector instances had a default cache size of 1, which meant having to create unique selector instances per component when sharing selectors that took varying arguments like IDs. weakMapMemoize memoizes based on all arguments, so it eliminated the extra setup work and just stores all cached values.

However, that behavior can also effectively turn into a memory leak depending on what state and arguments are passed in and how they're used in the UI.

weakMapMemoize now accepts a maxSize option that bounds this growth:

const getVisibleItems = weakMapMemoize(
  (items, from, to) => items.slice(from, to),
  { maxSize: 100 }
)

This shares the same maxSize option name as the earlier lruMemoize, but has different behavior. Rather than an LRU eviction, this is a "generational" swap (kind of like a double buffer). When the cache hits its max size, it's swapped out with an empty version and starts over at 0 entries. So, there's effectively at most 2 * maxSize items in memory at any time. If maxSize is not enabled, there's no additional logic or performance overhead. If you do need LRU-style behavior, use lruMemoize instead.

Note that bounding a createSelector selector requires passing maxSize in both memoizeOptions and argsMemoizeOptions, since the two memoization levels have separate caches. See the maxSize docs for details.

Thanks to @veksa for proposing this in PR #761 and providing memory test infrastructure that helped verify this behavior.

New cacheSizeCheck Dev-Mode Check

We've also added an additional dev-mode check alongside the existing inputStabilityCheck and identityFunctionCheck. If a memoized function has accumulated over 1000 values for the same args, it logs a warning with the function name and stack trace. By default this runs once per function. Unlike the other two checks, it can only be configured globally, via setGlobalDevModeChecks({ cacheSizeCheck: 'always' | 'once' | 'never' })

Performance Improvements

We've revamped our own performance benchmark suite to give better results with more precision and less noise. That's helped verify some additional performance improvements.

• weakMapMemoize now returns early on a cache hit instead of continuing through bookkeeping
• lruMemoize's cache lookup was simplified to eliminate unnecessary allocations

These are small wins on already-fast paths, but did show modest improvements.

Removal of unstable_autotrackMemoize

We've removed the experimental unstable_autotrackMemoize export. It was added in v5.0 as an experiment in Glimmer-style dependency tracking, never left unstable_, and as far as we can tell never saw real adoption. If you were using it, switch to weakMapMemoize (the default) or lruMemoize.

TypeScript Support

Our TypeScript support matrix is now 5.6 and up, matching DefinitelyTyped, and CI now tests against TS 6.0.

We've documented on the selector fields that a full cache reset requires clearing both memoization levels: selector.clearCache() plus selector.memoizedResultFunc.clearCache().

We improved types handling in cases where TS strict mode is off (but please migrate to strict behavior as soon as possible!)

Docs Improvements

The API docs got a structural overhaul: every API page now follows a consistent "API Reference / Usage Guide" layout, the dev-checks page was rewritten, and the docs reflect that weakMapMemoize is the default memoizer since v5. We've also added some additional usage guidance as well.

What's Changed

• Additional publishing hardening by @markerikson in #755
• Bump arethetypeswrong CLI to 0.18.3 to fix fflate tarball crash by @veksa in #764
• Don't widen an all-optional selector result to any by @veksa in #776
• Fix assorted memoization correctness issues by @markerikson in #777
• test: add benchmarks for cache fill cost and retained cache size by @veksa in #769
• test: paired hot-path benchmark for createSelector by @veksa in #770
• perf(createSelector): gather input selector results without extra allocations by @veksa in #772
• perf(createSelector): stop allocating dev-check bookkeeping on every call by @veksa in #773
• perf(weakMapMemoize): return on a cache hit instead of rewriting the node by @veksa in #771
• Improve perf harness by @markerikson in #778
• Optimize lruMemoize cache lookups by @markerikson in #779
• Add regression test for resultEqualityCheck receiving a cleared WeakRef by @veksa in #763
• Update test infra and add additional memory tests by @markerikson in #780
• Add weakMap cache size dev check and revamp docs by @markerikson in #782
• Add a maxSize option for weakMapMemoize by @markerikson in #783
• docs: clarify how to fully clear a selector's cache (#569) by @veksa in #760
• Remove experimental autoTrackMemoize by @markerikson in #784
• Add docs note on complex selectors and state by @markerikson in #785

Full Changelog: v5.2.0...v5.3.0

v5.2.0

This feature release improves tree-shaking and fixes a potential ref leak in weakMapMemoize, as well as restoring trusted publishing provenance.

Changelog

Code Improvements

We've restructured some of the internals to improve tree-shaking.

We've fixed a rare case that could lead to weakMapMemoize leaking a WeakRef.

Trusted Publishing Fixed

We had set up trusted publishing for Reselect a couple years ago and did some releases with that enabled, but at some point I did a follow-up release that still used the previous manual workflow, and that lost the trusted publishing provenance flag. We had recent issues requesting a new release with trusted publishing enabled again, so we've fixed that with this release.

What's Changed

• Fix leaking Ref in weakMapMemoize by @tomavos in #729
• Improve treeshakeability of build artifacts by @aryaemami59 in #721

Full Changelog: v5.1.1...v5.2.0

v5.1.1

This patch release fixes behavior of resultEqualityCheck in weakMapMemoize, fixes the case of lruMemoize being given a maxSize less than 1, and tweaks the internal implementation of lruMemoize. (We've also updated our general build tooling.)

Changelog

Bug fixes

Previously, providing the resultEqualityCheck option to weakMapMemoize resulted in it being called with empty objects as part of the initialization / dev check process. That could be an issue if your comparison function expected different values. We've updated the logic to avoid that, as well as improving a couple other perf aspects.

Previously, passing a maxSize < 1 to lruMemoize would result in it creating a larger cache. That's now fixed.

lruMemoize now uses a symbol for its NOT_FOUND value instead of a string.

What's Changed

• Ensure lruMemoize correctly memoizes when maxSize is set to a number less than 1 by @aryaemami59 in #698
• Fix resultEqualityCheck behavior in weakMapMemoize by @aryaemami59 in #699
• Update TypeScript to 5.4 by @aryaemami59 in #708
• Upgrade to Yarn 4 by @aryaemami59 in #705
• Fix: use unique value for NOT_FOUND by @romgrk in #709

Full Changelog: v5.1.0...v5.1.1

v5.1.0

This minor release:

• Adds a new createSelector.withTypes<RootState>() and createStructuredSelector.withTypes<RootState>() API
• Deprecates the TypedStructuredSelectorCreator type introduced in 5.0
• Aims to reduce false positives in identityFunctionCheck by only running if the output selector is passed one argument
• Fixes a bug with weakMapMemoize's resultEqualityCheck when used with a primitive result.

withTypes

Most commonly, selectors will accept the root state of a Redux store as their first argument. withTypes allows you to specify what that first argument will be ahead of creating the selector, meaning it doesn't have to be specified.

// previously
export const selectPostById = createSelector(
  [
    (state: RootState) => state.posts.entities,
    (state: RootState, id: number) => id,
  ],
  (entities, id) => entities[id],
);
// now
export const createAppSelector = createSelector.withTypes<RootState>();

export const selectPostById = createAppSelector(
  [(state) => state.posts.entities, (state, id: number) => id],
  (entities, id) => entities[id],
);

Known limitations

Due to a Typescript issue, inference of the output selector's parameters only works with withTypes when using an array of input selectors.

If using the variadic version, you can either wrap your input selectors in an array instance (as above), or annotate the parameters manually.

export const createAppSelector = createSelector.withTypes<RootState>();

export const selectPostById = createAppSelector(
  (state) => state.posts.entities, 
  (state, id: number) => id,
  // parameters cannot be inferred, so need annotating
  (entities: Record<number, Post>, id: number) => entities[id],
);

What's Changed

• Reduce identityFunctionCheck false positives by @Methuselah96 in #660
• Fix cut content inside TOC of docs by @aryaemami59 in #664
• Remove redundant Svg requires from components in docs by @aryaemami59 in #665
• Fix _lastResult.deref is not a function (it is undefined) in React Native and Expo applications by @aryaemami59 in #671
• Update getting-started.mdx by @cchaonie in #672
• Update createSelectorCreator.mdx with correct defaults by @lukeapage in #674
• Introduce pre-typed createSelector via createSelector.withTypes<RootState>() method by @aryaemami59 in #673
• Bump RTK and React-Redux to latest versions by @aryaemami59 in #676
• add publish job by @EskiMojo14 in #677
• Wrap up implementation of TypedStructuredSelectorCreator by @aryaemami59 in #667
• Introduce pre-typed createStructuredSelector via createStructuredSelector.ts.withTypes<RootState>() method by @aryaemami59 in #678
• Bump vitest to v1 by @aryaemami59 in #668

New Contributors

• @Methuselah96 made their first contribution in #660
• @cchaonie made their first contribution in #672

Full Changelog: v5.0.1...v5.1.0

v5.0.0

This major release:

• Switches createSelector to use a new weakMapMemoize method as the default memoizer
• Renames the existing defaultMemoize method to lruMemoize
• Adds new configuration options to createSelector
• Adds new development mode checks
• Updates the packaging for better ESM/CJS compatibility and modernizes the build output
• Makes significant updates to the TS types.

This release has breaking changes. (note: this points to v5.0.1, which contains a hotfix that was released prior to the announcement.)

This release is part of a wave of major versions of all the Redux packages: Redux Toolkit 2.0, Redux core 5.0, React-Redux 9.0, Reselect 5.0, and Redux Thunk 3.0.

For full details on all of the breaking changes and other significant changes to all of those packages, see the "Migrating to RTK 2.0 and Redux 5.0" migration guide in the Redux docs.

We have a new docs site! The Reselect docs are now at https://reselect.js.org.

Note

The Redux core, Reselect, and Redux Thunk packages are included as part of Redux Toolkit, and RTK users do not need to manually upgrade them - you'll get them as part of the upgrade to RTK 2.0. (If you're not using Redux Toolkit yet, please start migrating your existing legacy Redux code to use Redux Toolkit today!)

# RTK
npm install @reduxjs/toolkit
yarn add @reduxjs/toolkit

# Standalone
npm install reselect
yarn add reselect

Changelog

createSelector Uses weakMapMemoize By Default

Reselect's createSelector originally only had one memoization function, which has originally called defaultMemoize (and per below, is now renamed to lruMemoize). It's always used a customizable comparison method to compare each argument. Over time, we added more functionality, particularly in v4.1.0 where lruMemoize gained options for {memoize, maxSize, resultEqualityCheck}.

However, lruMemoize has limitations. The biggest one is that the default cache size is 1. This makes selector instances hard to reuse in scenarios like list items, which might call selectSomeValue(state, props.id), and thus never actually memoize due to changing arguments. There are workarounds, but they're cumbersome - using createSelectorCreator to create a customized createSelector function with a different memoization implementation, creating unique selector instances per component, or setting a fixed maxSize.

For 5.0, we added a new weakMapMemoize memoization function, which takes a different approach (as originally implemented in the React codebase). It uses an internal tree of cache nodes rather than a single value or a list of values. This gives weakMapMemoize an effectively infinite cache size!

We've done a fair amount of testing, and weakMapMemoize both performs faster and has more frequent cache hits than lruMemoize.

Given that, we've made the switch so that createSelector uses weakMapMemoize by default! This should result in better performance for Redux and React apps that use Reselect.

This is hopefully a mostly non-breaking change at the code level, and an overall improvement at the behavior level.

This is a breaking change. weakMapMemoize does not have an equalityCheck option or allow customizing the comparison behavior - it's entirely based on reference comparisons, since it uses WeakMap/Map internally. It also does not have a maxSize option, but does have resultEqualityCheck.

If you need to customize the overall equality comparison behavior, import and pass lruMemoize as the memoize and argsMemoize option!

Also, note that an "infinite cache size" from one point of view can be considered a "memory leak" for another point of view. The use of WeakMaps should mean that in most cases values do get garbage collected when the rest of the app no longer needs those, but there may be some scenarios with use of primitive keys that could lead to potential leaks. If this looks like it's happening for you, please compare behavior with lruMemoize instead, and file an issue report so we can investigate.

New / Improved createSelector Memoization Options

Originally, the only way to customize createSelector's behavior (such as using an alternate memoization function) was to first create a customized version via createSelectorCreator(memoizerFunction, memoizerOptions). This was typically used for creating use cases like deep equality comparisons with _.equal instead of shallow equality, as well as alternate memoizers that had a notion of cache size.

With Reselect 4.1.0, we added the ability to pass memoizer options directly to createSelector, and also updated defaultMemoize to accept several options such as a max cache size. This meant that you could call createSelector(...inputFns, outputFn, {memoizeOptions: {maxSize: 100}}), but you couldn't change the memoizer _function_ being used directly - that still required use of createSelectorCreator`.

Additionally, Reselect internally uses the provided memoizer function twice internally: once on the overall arguments passed to selectSomeValue(a, b, c), and a second time on the values extracted by the input functions such as state => state.a. There have been multiple issues over the years where users wanted to provide separate memoization functions for the arguments vs the extracted values, such as a reference equality check for the arguments and a shallow check for the extracted values.

With this release, you can now pass alternate memoizer functions directly to createSelector, and both createSelector and createSelectorCreator accept separate options for memoize and argsMemoize (along with any options for those):

const selectTodoIds = createSelector(
  (state: TodoState) => state.todos,
  todos => todos.map(({ id }) => id),
  {
    memoize: defaultMemoize,
    memoizeOptions: {
      resultEqualityCheck: (a, b) => a === b
    }
    argsMemoize: microMemoize,
    argsMemoizeOptions: { 
      isEqual: (a, b) => a === b 
    },
  }
)

This should mostly eliminate the need to use createSelectorCreator for customization. (You can still use it for encapsulation / reuse if you want to create many selectors with the same customization options.)

ESM/CJS Package Compatibility

The biggest theme of the Redux v5 and RTK 2.0 releases is trying to get "true" ESM package publishing compatibility in place, while still supporting CJS in the published package.

The primary build artifact is now an ESM file, dist/reselect.mjs. Most build tools should pick this up. There's also a CJS artifact, and a second copy of the ESM file named reselect.legacy-esm.js to support Webpack 4 (which does not recognize the exports field in package.json). Additionally, all of the build artifacts now live under ./dist/ in the published package.

Modernized Build Output

We now publish modern JS syntax targeting ES2020, including optional chaining, object spread, and other modern syntax. If you need to

Build Tooling

We're now building the package using https://github.com/egoist/tsup. We also now include sourcemaps for the ESM and CJS artifacts.

Dropping UMD Builds

Redux has always shipped with UMD build artifacts. These are primarily meant for direct import as script tags, such as in a CodePen or a no-bundler build environment.

We've dropped those build artifacts from the published package, on the grounds that the use cases seem pretty rare today.

There's now a reselect.browser.mjs file in the package that can be loaded from a CDN like Unpkg.

If you have strong use cases for us continuing to include UMD build artifacts, please let us know!

Dev Mode Checks

createSelector now does checks in development mode for common mistakes, like input selectors that always return new references, or result functions that immediately return their argument. These checks can be customized at selector creation or globally.

This is important, as an input selector returning a materially different result with the same parameters means that the output selector will never memoize correctly and be run unnecessarily, thus (potentially) creating a new result and causing rerenders.

const addNumbers = createSelector(
  // this input selector will always return a new reference when run
  // so cache will never be used
  (a, b) => ({ a, b }),
  ({ a, b }) => ({ total: a + b })
)
// instead, you should have an input selector for each stable piece of data
const addNumbersStable = createSelector(
  (a, b) => a,
  (a, b) => b,
  (a, b) => ({
    total: a + b,
  })
)

This is done the first time the selector is called, unless configured otherwise. See the Reselect docs on dev-mode checks for more details.

TypeScript Changes

We've dropped support for TS 4.6 and earlier, and our support matrix is now TS 4.7+.

The ParametricSelector and OutputParametricSelector types have been removed. Use Selector and OutputSelector instead.

The TS types have been updated to provide a better visual hover preview representation of a selector. It should now actually be previewed as "a function with attached fields", like:

const selectTodos: ((state: {
    todos: {
        id: number;
        title: string;
        description: string;
        completed: boolean;
    }[];
}) => number[]) & {
    clearCa...

v5.0.0-rc.1

This release candidate renames the original defaultMemoize function to lruMemoize, and improves dev check warnings to include a stack trace to help track down details. This release has breaking changes.

See the preview Redux Toolkit 2.0 + Redux core 5.0 Migration Guide for an overview of breaking changes in RTK 2.0 and Redux core.

npm install reselect@next

yarn add reselect@next

Changelog

Renaming defaultMemoize to lruMemoize

Reselect's createSelector originally only had one memoization function, which has always called defaultMemoize. In the previous v5.0.0-rc.0 release, we switched createSelector to use the new weakMapMemoize function as the default.

That meant that defaultMemoize was now badly named, because it isn't the default any more.

Given that, we've renamed defaultMemoize to lruMemoize, to better describe what it does.

This should not affect most Reselect users, since few apps actually customize selector setup. For those that do have references to defaultMemoize in the codebase, replace those with lruMemoize.

What's Changed

• Add stack to dev check warnings by @EskiMojo14 in #653
• Rename defaultMemoize to lruMemoize by @aryaemami59 in #654

Full Changelog: v5.0.0-rc.0...v5.0.0-rc.1

v5.0.0-rc.0

This release candidate switches createSelector to use weakMapMemoize as the default memoization method, adds a resultEqualityCheck option to weakMapMemoize, reworks the dev mode check setup and adds a dev-mode check for result functions that look like x => x, and makes some final types tweaks. This has breaking changes.

See the preview Redux Toolkit 2.0 + Redux core 5.0 Migration Guide for an overview of breaking changes in RTK 2.0 and Redux core.

npm install reselect@next

yarn add reselect@next

Changelog

createSelector Uses weakMapMemoize By Default

Reselect's createSelector originally only had one memoization function, which has always called defaultMemoize. It's always used a customizable comparison method to compare each argument. Over time, we added more functionality, particularly in v4.1.0 where defaultMemoize gained options for {memoize, maxSize, resultEqualityCheck}.

However, defaultMemoize has limitations. The biggest one is that the default cache size is 1. This makes selector instances hard to reuse in scenarios like list items, which might call selectSomeValue(state, props.id), and thus never actually memoize due to changing arguments. There are workarounds, but they're cumbersome - using createSelectorCreator to create a customized createSelector function with a different memoization implementation, creating unique selector instances per component, or setting a fixed maxSize.

For 5.0, we added a new weakMapMemoize memoization function, which takes a different approach. It uses an internal tree of cache nodes rather than a single value or a list of values. This gives weakMapMemoize an effectively infinite cache size!

We've done a fair amount of testing, and weakMapMemoize both performs faster and has more frequent cache hits than defaultMemoize.

Given that, we've made the switch so that createSelector uses weakMapMemoize by default! This should result in better performance for Redux and React apps that use Reselect.

This is hopefully a mostly non-breaking change at the code level, and an overall improvement at the behavior level.

This is a breaking change. weakMapMemoize does not have an equalityCheck option or allow customizing the comparison behavior - it's entirely based on reference comparisons, since it uses WeakMap/Map internally. It also does not have a maxSize option, but does have resultEqualityCheck.

If you need to customize the overall equality comparison behavior, import and pass defaultMemoize as the memoize and argsMemoize option!

Also, since defaultMemoize is no longer the actual "default" memoization function, we are considering a potential rename of defaultMemoize to something like lruMemoize to clarify the naming.

Dev-Mode Checks

Earlier, we added an inputStabilityCheck that checked for input selectors that accidentally return new references, with a globally exported override method.

In this release, we've added an additional dev mode check that looks for result functions that look like x => x - in other words, passing the input function result straight through. This is almost always a logic error. Either the input selectors are doing too much work, or the selector is just performing a straight lookup with no derived values and it should be a plain function instead of memoized.

Both checks are customizable on a per-selector-instance basis, and also controllable at a global level:

setGlobalDevModeChecks({
  inputStabilityCheck: 'always',
  identityFunctionCheck: 'never'
})

createSelector(
  [input1, input2],
  resultFn,
  {devModeChecks: {identityFunctionCheck: 'always'}}
)

What's Changed

• Update Docs for v5 changes. by @aryaemami59 in #636
• V5 final type changes by @aryaemami59 in #639
• Match resetRecomputations and resetDependencyRecomputations behavior to their types by @aryaemami59 in #646
• Try adding resultEqualityCheck to weakMapMemoize by @markerikson in #647
• Make EqualityFn slightly more type-safe. by @aryaemami59 in #651
• Switch createSelector to use weakMapMemoize by default by @markerikson in #649
• Add warning in development when a result function is x => x. by @aryaemami59 in #645

Full Changelog: v5.0.0-beta.1...v5.0.0-rc.0

v5.0.0-beta.1

This beta release improves the TS types so that hover previews of generated selectors have a much more readable format.

Note that we hope to release Redux Toolkit 2.0, Redux core 5.0, and React-Redux 9.0 by the start of December! (If we don't hit that, we'll aim for January, after the holidays.)

See the preview Redux Toolkit 2.0 + Redux core 5.0 Migration Guide for an overview of breaking changes in RTK 2.0 and Redux core.

npm install reselect@beta

yarn add reselect@beta

What's Changed

• Fix hover preview of generated selectors. by @aryaemami59 in #633

Full Changelog: v5.0.0-beta.0...v5.0.0-beta.1

v5.0.0-beta.0

This beta release updates createSelector to accept additional memoization functions and memoizer options directly (without needing to use createSelectorCreator first), renames an experimental memoizer to unstable_autotrackMemoizer for clarity, and drops support for TS 4.6 and earlier.

npm i reselect@beta

yarn add reselect@beta

This is part of the in-progress Redux Toolkit 2.0 beta work, and Reselect 5.0 will be included with Redux Toolkit 2.0 when it is released. Please try out RTK 2.0 beta and give us feedback!

Changelog

New / Improved createSelector Memoization Options

Originally, the only way to customize createSelector's behavior (such as using an alternate memoization function) was to first create a customized version via createSelectorCreator(memoizerFunction, memoizerOptions). This was typically used for creating use cases like deep equality comparisons with _.equal instead of shallow equality, as well as alternate memoizers that had a notion of cache size.

With Reselect 4.1.0, we added the ability to pass memoizer options directly to createSelector, and also updated defaultMemoize to accept several options such as a max cache size. This meant that you could call createSelector(...inputFns, outputFn, {memoizeOptions: {maxSize: 100}}), but you couldn't change the memoizer _function_ being used directly - that still required use of createSelectorCreator`.

Additionally, Reselect internally uses the provided memoizer function twice internally: once on the overall arguments passed to selectSomeValue(a, b, c), and a second time on the values extracted by the input functions such as state => state.a. There have been multiple issues over the years where users wanted to provide separate memoization functions for the arguments vs the extracted values, such as a reference equality check for the arguments and a shallow check for the extracted values.

With this release, you can now pass alternate memoizer functions directly to createSelector, and both createSelector and createSelectorCreator accept separate options for memoize and argsMemoize (along with any options for those):

const selectTodoIds = createSelector(
  (state: TodoState) => state.todos,
  todos => todos.map(({ id }) => id),
  {
    memoize: defaultMemoize,
    memoizeOptions: {
      resultEqualityCheck: (a, b) => a === b
    }
    argsMemoize: microMemoize,
    argsMemoizeOptions: { 
      isEqual: (a, b) => a === b 
    },
  }
)

This should mostly eliminate the need to use createSelectorCreator for customization. (You can still use it for encapsulation / reuse if you want to create many selectors with the same customization options.)

Thanks to @aryaemami59 for some incredibly comprehensive efforts reworking the internals of createSelector, our TS types, and the codebase structure in order to make this possible!

Other Changes

We've dropped support for TS 4.6 and earlier. Our current TS support matrix is TS 4.7+.

The experimental autotrackMemoize function has been renamed to unstable_autotrackeMemoize. It will still be exported, but it needs significant further testing before we consider it ready.

What's Changed

• Drop support for TS 4.6 and earlier, and update CI to use latest RTK betas by @markerikson in #630
• Allow passing in memoize functions directly to createSelector. by @aryaemami59 in #626
• Rename to unstable_autotrackMemoize and bump Vitest version by @markerikson in #631

Full Changelog: v5.0.0-alpha.2...v5.0.0-beta.0

v5.0.0-alpha.2

This alpha release updates createSelector to run an extra one-time check for input selector stability in development builds, and updates the build artifacts to include sourcemaps and a browser-ready ESM production bundle.

Changelog

Development Input Stability Checks

Reselect uses two levels of memoization. The first level checks if any of the actual arguments have changed, and the second sees if the values extracted by the input functions have changed.

If an input function always returns a new reference, like (state) => ({a: state.a, b: state.b}) or (state) => state.items.map(), that will cause the selector to never memoize properly. This is a bug, similar conceptually to always returning a new reference in useSelector(), or always including a new reference in useEffect's dependencies array.

Since this is a common mistake, we've added a development mode check to catch this. By default, createSelector will now run try executing the memoization function twice during the first call to the selector. If the result appears to be different, it will log a warning with the arguments and the two different sets of extracted input values.

You can configure this behavior in two ways: by passing an inputStabilityCheck option directly to createSelector, or by importing the global setInputStabilityCheckEnabled() function:

type StabilityCheck = 'always' | 'once' | 'never'

const unstableInput = (a: number, b: number) => ({ a, b })

// Create a selector that double-checks the inputs every time it runs
const selector = createSelector(
  unstableInput,
  ({ a, b }) => a + b,
  {inputStabilityCheck: 'always'}
)

// Set all selectors to never double-check by default (unless given a specific option)
import { setInputStabilityCheckEnabled } from 'reselect'

setInputStabilityCheckEnabled('never')

Build Artifact Changes

We now include a reselect.browser.mjs ESM artifact has been compiled for production settings and is ready for use as an ES module in browsers.

We also have updated the reselect.legacy-esm.js artifact to transpile syntax for compat with Webpack 4.

What's Changed

• Run input selectors twice, to check stability. by @EskiMojo14 in #612
• Build browser ESM and Webpack 4 compat artifacts, and add sourcemaps by @markerikson in #613

Full Changelog: v5.0.0-alpha.1...v5.0.0-alpha.2