Instead, every time you want to update an array, you’ll want to pass a new array to your state setting function. To do that, you can create a new array from the original array in your state by calling its non-mutating methods like filter() and map(). Then you can set your state to the resulting new array. There are some things you can’t do with the spread syntax and non-mutating methods like map() and filter() alone. For example, you may want to reverse or sort an array. The JavaScript reverse() and sort() methods are mutating the original array, so you can’t use them directly. If you want to change some or all items of the array, you can use map() to create a new array. The function you will pass to map can decide what to do with each item, based on its data or its index (or both).
handleChange: function(e) {
// 1. Make a shallow copy of the items
let items = [...this.state.items];
// 2. Make a shallow copy of the item you want to mutate
let item = {
...items[1]
};
// 3. Replace the property you're intested in
item.name = 'newName';
// 4. Put it back into our array. N.B. we *are* mutating the array here, but that's why we made a copy first
items[1] = item;
// 5. Set the state to our new copy
this.setState({
items
});
}, import { useState } from 'react';
let nextId = 0;
export default function List() {
const [name, setName] = useState('');
const [artists, setArtists] = useState([]);
return (
<>
<h1>Inspiring sculptors:</h1>
<input
value={name}
onChange={e => setName(e.target.value)}
/>
<button onClick={() => {
setName('');
artists.push({
id: nextId++,
name: name,
});
}}>Add</button>
<ul>
{artists.map(artist => (
<li key={artist.id}>{artist.name}</li>
))}
</ul>
</>
);
}
setArtists( // Replace the state [ // with a new array ...artists, // that contains all the old items { id: nextId++, name: name } // and one new item at the end ]);setArtists([{
id: nextId++,
name: name
}, ...artists // Put old items at the end]);import { useState } from 'react';
let initialArtists = [
{ id: 0, name: 'Marta Colvin Andrade' },
{ id: 1, name: 'Lamidi Olonade Fakeye'},
{ id: 2, name: 'Louise Nevelson'},
];
export default function List() {
const [artists, setArtists] = useState(
initialArtists
);
return (
<>
<h1>Inspiring sculptors:</h1>
<ul>
{artists.map(artist => (
<li key={artist.id}>
{artist.name}{' '}
<button onClick={() => {
setArtists(
artists.filter(a =>
a.id !== artist.id
)
);
}}>
Delete
</button>
</li>
))}
</ul>
</>
);
}
State can hold any kind of JavaScript value, including objects. But you shouldn’t change objects that you hold in the React state directly. Instead, when you want to update an object, you need to create a new one (or make a copy of an existing one), and then set the state to use that copy. Note that the ... spread syntax is “shallow”—it only copies things one level deep. This makes it fast, but it also means that if you want to update a nested property, you’ll have to use it more than once. To update a nested object, you need to create copies all the way up from the place you’re updating.
const [x, setX] = useState(0);setX(5);
const [position, setPosition] = useState({
x: 0,
y: 0
});import { useState } from 'react';
export default function MovingDot() {
const [position, setPosition] = useState({
x: 0,
y: 0
});
return (
<div
onPointerMove={e => {
position.x = e.clientX;
position.y = e.clientY;
}}
style={{
position: 'relative',
width: '100vw',
height: '100vh',
}}>
<div style={{
position: 'absolute',
backgroundColor: 'red',
borderRadius: '50%',
transform: `translate(${position.x}px, ${position.y}px)`,
left: -10,
top: -10,
width: 20,
height: 20,
}} />
</div>
);
}
onPointerMove = {
e => {
position.x = e.clientX;
position.y = e.clientY;
}
}To update an object in an array in React state: Use the map() method to iterate over the array. On each iteration, check if a certain condition is met. Update the object that satisfies the condition and return all other objects as is.25-Jul-2022 To update an object in a state array, call the map() method to iterate over the array and update the object that matches the condition. Copied! const updateObjectInArray = () => { setEmployees(current => current.23-Apr-2022 To update state in React components, we’ll use either the this. setState function or the updater function returned by the React. useState() Hook in class and function components, respectively.12-Jan-2022
With this article, we’ll look at some examples of how to address the How To Update State.Item[1] In State Using Setstate? React problem .
handleChange: function(e) {
// 1. Make a shallow copy of the items
let items = [...this.state.items];
// 2. Make a shallow copy of the item you want to mutate
let item = {
...items[1]
};
// 3. Replace the property you're intested in
item.name = 'newName';
// 4. Put it back into our array. N.B. we *are* mutating the array here, but that's why we made a copy first
items[1] = item;
// 5. Set the state to our new copy
this.setState({
items
});
},By way of numerous illustrations, we have demonstrated how to use code written to solve the How To Update State.Item[1] In State Using Setstate? React problem. How To Update State.Item[1] In State Using Setstate? React With Code Examples Programming CSS Bootstrap Golang Java Spring Javascript AngularJs BackboneJs Bootstrap EmberJs ExpressJs Flutter IonicJs JQuery NextJs NodeJs ReactJs Typescript VueJs PHP Cake PHP Codeigniter Drupal Symfony WordPress Yii Python Flask Rust Salesforce Swift Node Js Connect To Mongodb Using Mongoose With Code Examples
Hello everyone, in this post we will look at how to solve the How To Update State.Item[1] In State Using Setstate? React problem in the programming language.
handleChange: function(e) {
// 1. Make a shallow copy of the items
let items = [...this.state.items];
// 2. Make a shallow copy of the item you want to mutate
let item = {
...items[1]
};
// 3. Replace the property you're intested in
item.name = 'newName';
// 4. Put it back into our array. N.B. we *are* mutating the array here, but that's why we made a copy first
items[1] = item;
// 5. Set the state to our new copy
this.setState({
items
});
},The this.setState() method on the component instance is used to update the React state. It does a shallow merge, meaning that when you update one property in the state (e.g. list), the other properties in the state stay intact. The second question, which asks how to push an empty array in React state, involves manipulating the state with this.setState(). Let's say you want to empty the array on a button click. Then you can do it the following way: First, it is not allowed to use the array push method, because it mutates the array. It doesn't leave the array intact but changes it. Instead, there should be a new array created which is used to update the state.
Before we are going to manipulate a JavaScript array in React state, let's recap state in React shortly. State in React can be initialized in the constructor of a React component and afterward used by accessing it via the React component's class instance with the this object.
import React, { Component } from 'react';
class App extends Component { constructor(props) { super(props);
this.state = { list: [1, 2, 3], }; }
render() { return ( <div> <ul> {this.state.list.map(item => ( <li key={item}>{item}</li> ))} </ul> </div> ); }}
export default App;The first question can be answered with React's component constructor by simply initializing the state as as empty array:
import React, { Component } from 'react';
class App extends Component { constructor(props) { super(props);
this.state = { list: [], }; }
render() { return ( <div> <ul> {this.state.list.map(item => ( <li key={item}>{item}</li> ))} </ul> </div> ); }}
export default App;The map() method in the render() method, that is used to display the items of the array, does work, because it iterates over an empty array and thus returns no item for it. The iterator function in the map method isn't called. Let's say you would have a null state for your array instead, then you would have to have a fallback for your map method in your render method:
import React, { Component } from 'react';
class App extends Component { constructor(props) { super(props);
this.state = { list: null, }; }
render() { return ( <div> <ul> {(this.state.list || []).map(item => ( <li key={item}>{item}</li> ))} </ul> </div> ); }}
export default App;You set an empty array as React state for the component by having a clear method for it. Building on top of this example, the third question, which asks how to create an initial array state, can be answered too. You have already seen how an initial array is set to state in the component's constructor. How would you reinitialize the initial state again, basically a reset of the state, after you have manipulated the state already? You can extract the initial state and then set it again whenever you want.
import React, { Component } from 'react';
const list = [1, 2, 3];
class App extends Component { constructor(props) { super(props);
this.state = { list, }; }
onClearArray = () => { this.setState({ list: [] }); };
onResetArray = () => { this.setState({ list }); };
render() { return ( <div> <ul> {this.state.list.map(item => ( <li key={item}>{item}</li> ))} </ul>
<button type="button" onClick={this.onClearArray}> Clear Array </button>
<button type="button" onClick={this.onResetArray}> Reset Array </button> </div> ); }}
export default App;The state reset was only applied to the array. But you can apply it to your entire state too by extracting the whole initial state object.
import React, { Component } from 'react';
const INITIAL_STATE = { list: [1, 2, 3],};
class App extends Component { constructor(props) { super(props);
this.state = INITIAL_STATE; }
onClearArray = () => { this.setState({ list: [] }); };
onResetArray = () => { this.setState({ ...INITIAL_STATE }); };
render() { return ( <div> <ul> {this.state.list.map(item => ( <li key={item}>{item}</li> ))} </ul>
<button type="button" onClick={this.onClearArray}> Clear Array </button>
<button type="button" onClick={this.onResetArray}> Reset Array </button> </div> ); }}
export default App;The state object of a component may contain multiple attributes and React allows using setState() function to update only a subset of those attributes as well as using multiple setState() methods to update each attribute value independently. Syntax: We can use setState() to change the state of the component directly as well as through an arrow function. setState is asynchronous call means if synchronous call get called it may not get updated at right time like to know current value of object after update using setState it may not get give current updated value on console. To get some behavior of synchronous need to pass function instead of object to setState.
setState({
stateName: updatedStateValue
})
// OR
setState((prevState) => ({
stateName: prevState.stateName + 1
}))Step 1: Create a React application using the following command:
npx create - react - app foldername
Step 2: After creating your project folder i.e. foldername, move to it using the following command:
cd foldername
As you can see, we are passing an object to setState(). This object contains the part of the state we want to update which, in this case, is the value of greeting. React takes this value and merges it into the object that needs it. It’s just like the button component asks what it should use for updating the value of greeting and setState() responds with an answer.
import React, { Component } from 'react'
class App extends Component {
constructor(props){
super(props)
// Set initial state
this.state = {greeting :
'Click the button to receive greetings'}
// Binding this keyword
this.updateState = this.updateState.bind(this)
}
updateState(){
// Changing state
this.setState({greeting :
'PretagcodeForPretagcode welcomes you !!'})
}
render(){
return (
<div>
<h2>Greetings Portal</h2>
<p>{this.state.greeting}</p>
{/* Set click handler */}
<button onClick={this.updateState}>
Click me!
</button>
</div>
)
}
}
export default App;
Step to Run Application: Run the application using the following command from the root directory of the project:
npm start
We must know how to update state using React and choose the appropriate state hook for our purposes. We might choose useReducer, useState, or a third-party state management library like Redux. In other words, if we update state with plain JavaScript and not setState, it will not trigger a re-render and React will not display those (invalid) changes in state to our user. This pattern is commonly used to update state in class-based components, but this does not work with the useState hook. State updates with useState's setState function are not automatically merged.
A great advantage of the useState hook is that we are able to call it as many times as we like to use as many state variables as we need.
In this example, we have a basic form with an email and password input. We are managing the email and password state as individual state variables:
import React from "react";
export default function App() {
const [email, setEmail] = React.useState("");
const [password, setPassword] = React.useState("");
return (
<form>
<input
name="email"
type="email"
placeholder="Email"
onChange={(e) => setEmail(e.target.value)}
/>
<input
name="password"
type="password"
placeholder="Password"
onChange={(e) => setPassword(e.target.value)}
/>
<button type="submit">Submit</button>
</form>
);
}How do we appropriately update state with the setState function when it is an object?
If we were to use a generic event handler that is connected to the onChange prop of each of our form's inputs, it would look something like this:
import React from "react";
export default function App() {
const [state, setState] = React.useState({
email: '',
password: ''
})
function handleInputChange(e) {
setState({
[e.target.name]: e.target.value
})
}
return (
<form>
<input
name="email"
type="email"
onChange={handleInputChange}
/>
<input
name="password"
type="password"
onChange={handleInputChange}
/>
<button type="submit">Submit</button>
</form>
);
}Since the previous state is not automatically merged into the new state object, we must manually merge our state object with its previous properties using the object spread operator:
import React from "react";
export default function App() {
const [state, setState] = React.useState({
email: '',
password: ''
})
function handleInputChange(e) {
setState({
// spread in previous state with object spread operator
...state,
[e.target.name]: e.target.value
})
}
return (
<form>
<input
name="email"
type="email"
onChange={handleInputChange}
/>
<input
name="password"
type="password"
onChange={handleInputChange}
/>
<button type="submit">Submit</button>
</form>
);
}One other thing to note here is that there is technically a way to manage state without causing a re-render. We can do so with a hook that most people don't view as being a stateful React hook – useRef.
useRef can be used to store any value on its .current property. In other words, if we wanted to make a simple counter with useRef and update a count value that we stored on it, even if we update its value, it would not show the correct count after the initial render because doing so does not trigger a re-render:
import React from "react";
export default function App() {
const countRef = React.useRef(0);
function handleAddOne() {
countRef.current += 1;
}
return (
<>
<h1>Count: {countRef.current}</h1>
{/* clicking this will not change display count */}
<button onClick={handleAddOne}>+ 1</button>
</>
);
}The other reason for this, aside from our React application not working properly, is that it violates a core principle of React. This is the concept of immutability.
State updates should always be immutable. This means we shouldn't make our own changes or mutate the data stored in our state variables. Doing so makes our state unpredictable and can cause unintended problems in our application that are hard to debug.
import React from 'react';
export default function App() {
const [count, setCount] = React.useState(0);
// Don't assign state to new (non-state) variables
const newCount = count;
// Don't directly mutate state
const countPlusOne = count + 1;
return (
<>
<h1>Count: {count}</h1>
</>
);
}If we want to use arrays or objects in our React state, we have to create a copy of the value before modifying it. This is a cheat sheet on how to do add, remove, and update items in an array or object within the context of managing React state. Just wrote How to update an array of objects in React state We can also use the spread operator to create copy and append an item with the following syntax: Edit: Thanks for this cheatsheet, nice to have it open when fighting with array states.
const [todos, setTodos] = useState([]);const handleAdd = (todo) => {
const newTodos = todos.slice();
newTodos.push(todo);
setTodos(newTodos);
}const handleAdd = (todo) => {
const newTodos = [...todos];
newTodos.push(todo);
setTodos(newTodos);
}const handleRemove = (todo) => {
const newTodos = todos.filter((t) => t !== todo);
setTodos(newTodos);
}const handleUpdate = (index, todo) => {
const newTodos = [...todos];
newTodos[index] = todo;
setTodos(newTodos);
}The function gets called with the current state and can be used to update a specific object in the state array. To update an object in an array in React state: Use the map() method to iterate over the array. On each iteration, check if a certain condition is met.
Copied!import {useState} from 'react';
const App = () => {
const initialState = [
{id: 1, country: 'Austria'},
{id: 2, country: 'Belgium'},
{id: 3, country: 'Canada'},
];
const [data, setData] = useState(initialState);
const updateState = () => {
const newState = data.map(obj => {
// 👇️ if id equals 2, update country property
if (obj.id === 2) {
return {...obj, country: 'Denmark'};
}
// 👇️ otherwise return object as is
return obj;
});
setData(newState);
};
return (
<div>
<button onClick={updateState}>Update state</button>
{data.map(obj => {
return (
<div key={obj.id}>
<h2>id: {obj.id}</h2>
<h2>country: {obj.country}</h2>
<hr />
</div>
);
})}
</div>
);
};
export default App;
Copied!import {useState} from 'react';
const App = () => {
const initialState = [
{id: 1, country: 'Austria'},
{id: 2, country: 'Belgium'},
{id: 3, country: 'Canada'},
];
const [data, setData] = useState(initialState);
const updateState = () => {
// 👇️ passing function to setData method
setData(prevState => {
const newState = prevState.map(obj => {
// 👇️ if id equals 2, update country property
if (obj.id === 2) {
return {...obj, country: 'Denmark'};
}
// 👇️ otherwise return object as is
return obj;
});
return newState;
});
};
return (
<div>
<button onClick={updateState}>Update state</button>
{data.map(obj => {
return (
<div key={obj.id}>
<h2>id: {obj.id}</h2>
<h2>country: {obj.country}</h2>
<hr />
</div>
);
})}
</div>
);
};
export default App;