Redux Toolkit · Function

isActionCreator

isActionCreator is a configurable predicate used by Redux Toolkit's action-creator invariant middleware. It identifies values that should be treated as action creators, including custom action-creator shapes.

How it works

isActionCreator is a configurable type-guard predicate for action-creator invariant middleware. It receives an unknown value and returns whether that value should be treated as an action creator; the middleware can then warn when the creator itself is dispatched instead of its returned action.

Syntax

isActionCreator?: (action: unknown) => action is Function & { type?: unknown }

Parameters

Name	Type	Description
`action`	`unknown`	The value to classify as an action creator; its runtime type is unknown.

Returns

A boolean type predicate: action is Function & { type?: unknown } in the documented option signature.

Key points

The option is a callback that decides whether an arbitrary runtime value should count as an action creator.
Default detection targets standard Redux Toolkit action creators by requiring a function together with both static type and match members.
A custom predicate can deliberately broaden detection to every function carrying a static type property.
Configure the predicate by supplying it in the options passed to createActionCreatorInvariantMiddleware.
Most applications can rely on the middleware included by getDefaultMiddleware rather than constructing it directly.

Common mistakes

Pass the result of calling the action creator to dispatch; dispatching the function itself is the mistake this middleware is designed to detect.
A custom predicate can broaden or change detection, so ensure its criteria accurately identify the action creators used by the application.

Example: Custom action-creator predicate

import { configureStore, createActionCreatorInvariantMiddleware } from '@reduxjs/toolkit'
import reducer from './reducer'

const isActionCreator = (
  action: unknown,
): action is Function & { type: unknown } =>
  typeof action === 'function' && 'type' in action

const middleware = createActionCreatorInvariantMiddleware({ isActionCreator })

const store = configureStore({
  reducer,
  middleware: () => [middleware],
})

export default store

This custom predicate treats any function with a static type property as an action creator, allowing the invariant middleware to detect when such a function is dispatched without being called.

When to use it

Broaden middleware detection

An application uses action creators with a static type property but without the standard match method.

Support custom creator shapes

A project defines a custom action-creator convention that the default predicate does not recognize.

Catch dispatch mistakes

A team wants warnings when a creator function is dispatched directly instead of being invoked.

Interview questions

QWhat is isActionCreator?

isActionCreator is a predicate used by the action-creator invariant middleware to recognize values that represent action creators. Its return type is a TypeScript type predicate, narrowing a matching value to a function with an optional type property.

Follow-up: Why does the default recognize both type and match? Redux Toolkit action creators expose a static type string and a match() method, so checking both identifies the standard action-creator shape.

QWhat does the predicate inspect?

The function receives an unknown value and should return a boolean. The documented custom example identifies values that are functions and have a type property, returning true for those values.

Follow-up: How would you include creators without a type property? Change the predicate to test the actual distinguishing characteristic of those creators, while preserving a sound type predicate.

QHow is a custom predicate configured?

Pass the predicate through the isActionCreator option when creating action-creator invariant middleware. This is useful when the default recognition rules do not cover an application's action-creator conventions.

Follow-up: Is explicit middleware creation normally required? Usually not: getDefaultMiddleware already includes the action-creator check middleware.

createActionCreatorInvariantMiddleware
getDefaultMiddleware
createAction
createSlice

Source: Action Creator Middleware - Redux Toolkit. Checked 2026-10-15.