reduxjs/react-redux · error

You must pass a function as an equality function to useSelec

Error message

You must pass a function as an equality function to useSelector

What it means

When using the options form useSelector(selector, { equalityFn }), the equalityFn field must be a function (e.g. shallowEqual). Development builds throw this when it is a non-function truthy value such as a string or object.

Source

Thrown at src/hooks/useSelector.ts:156

  const useSelector = <TState, Selected>(
    selector: (state: TState) => Selected,
    equalityFnOrOptions:
      | EqualityFn<NoInfer<Selected>>
      | UseSelectorOptions<NoInfer<Selected>> = {},
  ): Selected => {
    const { equalityFn = refEquality } =
      typeof equalityFnOrOptions === 'function'
        ? { equalityFn: equalityFnOrOptions }
        : equalityFnOrOptions
    if (process.env.NODE_ENV !== 'production') {
      if (!selector) {
        throw new Error(`You must pass a selector to useSelector`)
      }
      if (typeof selector !== 'function') {
        throw new Error(`You must pass a function as a selector to useSelector`)
      }
      if (typeof equalityFn !== 'function') {
        throw new Error(
          `You must pass a function as an equality function to useSelector`,
        )
      }
    }

    const reduxContext = useReduxContext()

    const { store, subscription, getServerState } = reduxContext

    const firstRun = React.useRef(true)

    const wrappedSelector = React.useCallback<typeof selector>(
      {
        [selector.name](state: TState) {
          const selected = selector(state)
          if (process.env.NODE_ENV !== 'production') {
            const { devModeChecks = {} } =
              typeof equalityFnOrOptions === 'function'

View on GitHub (pinned to 16f1a91eb2)

Solutions

1. Import a real comparator: import { shallowEqual } from 'react-redux' and pass it directly
2. Or use the shorthand: useSelector(fn, shallowEqual)
3. Remove the equalityFn field if the default reference equality is acceptable

Example fix

// before
useSelector(sel, { equalityFn: 'shallow' })
// after
import { shallowEqual } from 'react-redux'
useSelector(sel, { equalityFn: shallowEqual })

import { shallowEqual } from 'react-redux'
const eq = shallowEqual // must be a function
if (process.env.NODE_ENV !== 'production' && typeof eq !== 'function') {
  throw new TypeError('equalityFn must be a function, e.g. shallowEqual')
}

function isEqualityFn(v: unknown): v is (a: any, b: any) => boolean {
  return typeof v === 'function' && v.length >= 2;
}

When it happens

Trigger: useSelector(fn, { equalityFn: 'shallow' }) or passing a misremembered constant/flag instead of a comparison function.

Common situations: Misreading docs and passing a string enum describing the comparison instead of the function import from react-redux.

Related errors

• You must pass a selector to useSelector
• You must pass a function as a selector to useSelector
• could not find react-redux context value; please ensure the
• You must pass a component to the function returned by connec
• You must pass a valid React context consumer as `props.conte

AI-assisted analysis of reduxjs/react-redux@16f1a91eb2 (2026-08-31). Data as JSON: /api/errors/1b14d38c7db5ee1d. Report an issue: GitHub.