Immutable Update Patterns

The key to updating nested data is that every level of nesting must be copied and updated appropriately. This is often a difficult concept for those learning Redux, and there are some specific problems that frequently occur when trying to update nested objects. These lead to accidental direct mutation, and should be avoided.

Common Mistake #1: New variables that point to the same objects

Defining a new variable does not create a new actual object - it only creates another reference to the same object. An example of this error would be:

This function does correctly return a shallow copy of the top-level state object, but because the variable was still pointing at the existing object, the state was directly mutated.

Common Mistake #2: Only making a shallow copy of one level

Another common version of this error looks like this:

function updateNestedState(state, action) {
  // Problem: this only does a shallow copy!
  let newState = { ...state }
  // ERROR: nestedState is still the same object!
  newState.nestedState.nestedField = action.data
  return newState
}

Doing a shallow copy of the top level is not sufficient - the nestedState object should be copied as well.

Correct Approach: Copying All Levels of Nested Data

Unfortunately, the process of correctly applying immutable updates to deeply nested state can easily become verbose and hard to read. Here's what an example of updating state.first.second[someId].fourth might look like:

function updateVeryNestedField(state, action) {
  return {
    ...state,
    first: {
      ...state.first,
        ...state.first.second,
        [action.someId]: {
          ...state.first.second[action.someId],
          fourth: action.someValue
        }
      }
  }
}

Normally, a Javascript array's contents are modified using mutative functions like push, unshift, and splice. Since we don't want to mutate state directly in reducers, those should normally be avoided. Because of that, you might see "insert" or "remove" behavior written like this:

However, remember that the key is that the original in-memory reference is not modified. As long as we make a copy first, we can safely mutate the copy. Note that this is true for both arrays and objects, but nested values still must be updated using the same rules.

This means that we could also write the insert and remove functions like this:

function insertItem(array, action) {
  let newArray = array.slice()
  newArray.splice(action.index, 0, action.item)
  return newArray
}
function removeItem(array, action) {
  let newArray = array.slice()
  newArray.splice(action.index, 1)
  return newArray
}

The remove function could also be implemented as:

function removeItem(array, action) {
  return array.filter((item, index) => index !== action.index)

Updating one item in an array can be accomplished by using Array.map, returning a new value for the item we want to update, and returning the existing values for all other items:

Because writing immutable update code can become tedious, there are a number of utility libraries that try to abstract out the process. These libraries vary in APIs and usage, but all try to provide a shorter and more succinct way of writing these updates. For example, makes immutable updates a simple function and plain JavaScript objects:

var usersState = [{ name: 'John Doe', address: { city: 'London' } }]
var newState = immer.produce(usersState, draftState => {
  draftState[0].name = 'Jon Doe'
  //nested update similar to mutable way
})

state = dotProp.set(state, `todos.${index}.complete`, true)

Others, like immutability-helper (a fork of the now-deprecated React Immutability Helpers addon), use nested values and helper functions:

They can provide a useful alternative to writing manual immutable update logic.

A list of many immutable update utilities can be found in the section of the Redux Addons Catalog.

Our package includes a createReducer utility that uses Immer internally.Because of this, you can write reducers that appear to "mutate" state, but the updates are actually applied immutably.

This allows immutable update logic to be written in a much simpler way. Here's what the might look like using createReducer:

import { createReducer } from 'redux-starter-kit'
const initialState = {
  first: {
    second: {
      id1: { fourth: 'a' },
      id2: { fourth: 'b' }
    }
  }
}
const reducer = createReducer(initialState, {
  UPDATE_ITEM: (state, action) => {
    state.first.second[action.someId].fourth = action.someValue
  }

This is clearly much shorter and easier to read. However, this only works correctly if you are using the "magic"createReducer function from Redux Starter Kit that wraps this reducer in Immer's produce function.If this reducer is used without Immer, it will actually mutate the state!. It's also not obvious just bylooking at the code that this function is actually safe and updates the state immutably. Please make sure you understandthe concepts of immutable updates fully. If you do use this, it may help to add some comments to your code that explainyour reducers are using Redux Starter Kit and Immer.