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[]) & {
    clearCache: () => void;
    resultsCount: () => number;
    resetResultsCount: () => void;
} & {
    lastResult: () => number[];
    recomputations: () => number;
    resetRecomputations: () => void;
    dependencyRecomputations: () => number;
    resetDependencyRecomputations: () => void;
    // snip additional attached functions
} 

Other Changes

Selectors now have a dependencyRecomputions method that returns how many times the dependency memoizer recalculated, and a resetDependencyRecomputations method that resets that value.

Similarly, both weakMapMemoize and lruMemoize now have resultsCount and resetResultsCount methods that count how many times they actually calculated new values. This should be equal to the number of outer recomputations, unless you have passed in resultEqualityCheck as an option, in which case it only counts times a new actual reference was returned.

Huge thanks to @aryaemami59 for some incredibly comprehensive efforts reworking the internals of createSelector, our TS types, and the codebase structure in order to make all these changes possible!

What's Changed

• Upgrade all build tooling for v5 alpha by @markerikson in https://github.com/reduxjs/reselect/pull/603
• Add experimental new memoizers: autotrack and weakmap by @markerikson in https://github.com/reduxjs/reselect/pull/605
• Run input selectors twice, to check stability. by @EskiMojo14 in https://github.com/reduxjs/reselect/pull/612
• Build browser ESM and Webpack 4 compat artifacts, and add sourcemaps by @markerikson in https://github.com/reduxjs/reselect/pull/613
• Drop support for TS 4.6 and earlier, and update CI to use latest RTK betas by @markerikson in https://github.com/reduxjs/reselect/pull/630
• Allow passing in memoize functions directly to createSelector. by @aryaemami59 in https://github.com/reduxjs/reselect/pull/626
• Rename to unstable_autotrackMemoize and bump Vitest version by @markerikson in https://github.com/reduxjs/reselect/pull/631
• Fix hover preview of generated selectors. by @aryaemami59 in https://github.com/reduxjs/reselect/pull/633
• V5 final type changes by @aryaemami59 in https://github.com/reduxjs/reselect/pull/639
• Match resetRecomputations and resetDependencyRecomputations behavior to their types by @aryaemami59 in https://github.com/reduxjs/reselect/pull/646
• Add benchmarks to test different memoization methods by @aryaemami59 in https://github.com/reduxjs/reselect/pull/640
• Try adding resultEqualityCheck to weakMapMemoize by @markerikson in https://github.com/reduxjs/reselect/pull/647
• Make EqualityFn slightly more type-safe. by @aryaemami59 in https://github.com/reduxjs/reselect/pull/651
• Switch createSelector to use weakMapMemoize by default by @markerikson in https://github.com/reduxjs/reselect/pull/649
• Add warning in development when a result function is x => x. by @aryaemami59 in https://github.com/reduxjs/reselect/pull/645
• Add stack to dev check warnings by @EskiMojo14 in https://github.com/reduxjs/reselect/pull/653
• Rename defaultMemoize to lruMemoize by @aryaemami59 in https://github.com/reduxjs/reselect/pull/654
• Fix WeakRef check by @markerikson in https://github.com/reduxjs/reselect/pull/657

Full Changelog: https://github.com/reduxjs/reselect/compare/v4.1.8...v5.0.1