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