How does importing a reducer in redux toolkit work?

Question

So I'm reading through the docs for rtk , the example shown is after creating a slice with this code:

import { createSlice } from '@reduxjs/toolkit'

const initialState = {
  value: 0,
}

export const counterSlice = createSlice({
  name: 'counter',
  initialState,
  reducers: {
    increment: (state) => {
      // Redux Toolkit allows us to write "mutating" logic in reducers. It
      // doesn't actually mutate the state because it uses the Immer library,
      // which detects changes to a "draft state" and produces a brand new
      // immutable state based off those changes
      state.value += 1
    },
    decrement: (state) => {
      state.value -= 1
    },
    incrementByAmount: (state, action) => {
      state.value += action.payload
    },
  },
})

// Action creators are generated for each case reducer function
export const { increment, decrement, incrementByAmount } = counterSlice.actions

export default counterSlice.reducer

and then importing the reducer in the store like this :

import { configureStore } from '@reduxjs/toolkit'
import counterReducer from '../features/counter/counterSlice'

export const store = configureStore({
  reducer: {
    counter: counterReducer,
  },
})

But what I'm seeing from the import is that it's importing counterReducer although there's no counterReducer export in the slice file , is that a Redux Toolkit feature or a React feature or a JavaScript feature that I'm not aware of?

Answer 1

It's a "default export" and "default import".

You notice that all exports have a name - except for

export default counterSlice.reducer

In the other file, you can see two different notations:

// has braces - imports the named export `configureStore` from `@reduxjs/toolkit`
import { configureStore } from '@reduxjs/toolkit'
// has no braces - imported the default export from the file `../features/counter/counterSlice` and gives it the name `counterReducer` for this file.
import counterReducer from '../features/counter/counterSlice'

Answer 2

It's an ES Modules feature, so it's a JavaScript thing. When you export something with default keyword, you can import it with whatever name you want and without {} . In slice file, there is this export:

export default counterSlice.reducer

And that you can import it with the name you want, they choose to call it counterReducer :

import counterReducer from '../features/counter/counterSlice'

It's the different between named exports and default exports . To know more about it, visit the doc for export statement or JavaScript modules on MDN.