[javascript] How to update single value inside specific array item in redux

I have an issue where re-rendering of state causes ui issues and was suggested to only update specific value inside my reducer to reduce amount of re-rendering on a page.

this is example of my state

 name: "some name",
 subtitle: "some subtitle",
 contents: [
   {title: "some title", text: "some text"},
   {title: "some other title", text: "some other text"}

and I am currently updating it like this

   return { ...state, contents: action.payload }

where action.payload is a whole array containing new values. But now I actually just need to update text of second item in contents array, and something like this doesn't work

   return { ...state, contents[1].text: action.payload }

where action.payload is now a text I need for update.

This question is related to javascript reactjs redux

The answer is

This is how I did it for one of my projects:

const markdownSaveActionCreator = (newMarkdownLocation, newMarkdownToSave) => ({
  saveLocation: newMarkdownLocation,
  savedMarkdownInLocation: newMarkdownToSave  

const markdownSaveReducer = (state = MARKDOWN_SAVED_ARRAY_DEFAULT, action) => {
  let objTemp = {
    saveLocation: action.saveLocation, 
    savedMarkdownInLocation: action.savedMarkdownInLocation

  switch(action.type) {
        state.map(i => {
          if (i.saveLocation === objTemp.saveLocation) {
            return Object.assign({}, i, objTemp);
          return i;
      return state;

You don't have to do everything in one line:

  const newState = { ...state };
  newState.contents = 
      {title: newState.contnets[1].title, text: action.payload}
  return newState

I'm afraid that using map() method of an array may be expensive since entire array is to be iterated. Instead, I combine a new array that consists of three parts:

  • head - items before the modified item
  • the modified item
  • tail - items after the modified item

Here the example I've used in my code (NgRx, yet the machanism is the same for other Redux implementations):

// toggle done property: true to false, or false to true

function (state, action) {
    const todos = state.todos;
    const todoIdx = todos.findIndex(t => t.id === action.id);

    const todoObj = todos[todoIdx];
    const newTodoObj = { ...todoObj, done: !todoObj.done };

    const head = todos.slice(0, todoIdx - 1);
    const tail = todos.slice(todoIdx + 1);
    const newTodos = [...head, newTodoObj, ...tail];

Very late to the party but here is a generic solution that works with every index value.

  1. You create and spread new array from the old array up to the index you want to change.

  2. Add the data you want.

  3. Create and spread new array from the index you wanted to change to the end of the array

let index=1;// probabbly action.payload.id
   return { 
       contents: [
          {title: "some other title", text: "some other text"},


I have made a small module to simplify the code, so you just need to call a function:

   return {
       contents: insertIntoArray(state.contents,index, {title: "some title", text: "some text"})

For more examples, take a look at the repository

function signature:


In my case I did something like this, based on Luis's answer:

...State object...
userInfo = {
name: '...',

...Reducer's code...
return {
  userInfo: {
    // I'm sending the arguments like this: changeInfo({ id: e.target.id, value: e.target.value }) and use them as below in reducer!
    [action.data.id]: action.data.value,

I believe when you need this kinds of operations on your Redux state the spread operator is your friend and this principal applies for all children.

Let's pretend this is your state:

const state = {
    houses: {
        gryffindor: {
          points: 15
        ravenclaw: {
          points: 18
        hufflepuff: {
          points: 7
        slytherin: {
          points: 5

And you want to add 3 points to Ravenclaw

const key = "ravenclaw";
  return {
    ...state, // copy state
    houses: {
      ...state.houses, // copy houses
      [key]: {  // update one specific house (using Computed Property syntax)
        ...state.houses[key],  // copy that specific house's properties
        points: state.houses[key].points + 3   // update its `points` property

By using the spread operator you can update only the new state leaving everything else intact.

Example taken from this amazing article, you can find almost every possible option with great examples.

You can use map. Here is an example implementation:

   return { 
       contents: state.contents.map(
           (content, i) => i === 1 ? {...content, text: action.payload}
                                   : content

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