I want to add an element to the end of a state array, is this the correct way to do it?
this.state.arrayvar.push(newelement);
this.setState({ arrayvar:this.state.arrayvar });I’m concerned that modifying the array in-place with push might cause trouble – is it safe?
The alternative of making a copy of the array, and setStateing that seems wasteful.
Answers:
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Method 1
The React docs says:
Treat this.state as if it were immutable.
Your push will mutate the state directly and that could potentially lead to error prone code, even if you are “resetting” the state again afterwards. F.ex, it could lead to that some lifecycle methods like componentDidUpdate won’t trigger.
The recommended approach in later React versions is to use an updater function when modifying states to prevent race conditions:
this.setState(prevState => ({
arrayvar: [...prevState.arrayvar, newelement]
}))The memory “waste” is not an issue compared to the errors you might face using non-standard state modifications.
Alternative syntax for earlier React versions
You can use concat to get a clean syntax since it returns a new array:
this.setState({
arrayvar: this.state.arrayvar.concat([newelement])
})In ES6 you can use the Spread Operator:
this.setState({
arrayvar: [...this.state.arrayvar, newelement]
})Method 2
Easiest, if you are using ES6.
initialArray = [1, 2, 3]; newArray = [ ...initialArray, 4 ]; // --> [1,2,3,4]
New array will be [1,2,3,4]
to update your state in React
this.setState({
arrayvar:[...this.state.arrayvar, newelement]
});Learn more about array destructuring
Method 3
The simplest way with ES6:
this.setState(prevState => ({
array: [...prevState.array, newElement]
}))Method 4
React may batch updates, and therefore the correct approach is to provide setState with a function that performs the update.
For the React update addon, the following will reliably work:
this.setState( state => update(state, {array: {$push: [4]}}) );or for concat():
this.setState( state => ({
array: state.array.concat([4])
}));The following shows what https://jsbin.com/mofekakuqi/7/edit?js,output as an example of what happens if you get it wrong.
The setTimeout() invocation correctly adds three items because React will not batch updates within a setTimeout callback (see https://groups.google.com/d/msg/reactjs/G6pljvpTGX0/0ihYw2zK9dEJ).
The buggy onClick will only add “Third”, but the fixed one, will add F, S and T as expected.
class List extends React.Component {
constructor(props) {
super(props);
this.state = {
array: []
}
setTimeout(this.addSome, 500);
}
addSome = () => {
this.setState(
update(this.state, {array: {$push: ["First"]}}));
this.setState(
update(this.state, {array: {$push: ["Second"]}}));
this.setState(
update(this.state, {array: {$push: ["Third"]}}));
};
addSomeFixed = () => {
this.setState( state =>
update(state, {array: {$push: ["F"]}}));
this.setState( state =>
update(state, {array: {$push: ["S"]}}));
this.setState( state =>
update(state, {array: {$push: ["T"]}}));
};
render() {
const list = this.state.array.map((item, i) => {
return <li key={i}>{item}</li>
});
console.log(this.state);
return (
<div className='list'>
<button onClick={this.addSome}>add three</button>
<button onClick={this.addSomeFixed}>add three (fixed)</button>
<ul>
{list}
</ul>
</div>
);
}
};
ReactDOM.render(<List />, document.getElementById('app'));Method 5
If you are using functional components in React
const [cars, setCars] = useState([{
name: 'Audi',
type: 'sedan'
}, {
name: 'BMW',
type: 'sedan'
}])
...
const newCar = {
name: 'Benz',
type: 'sedan'
}
const updatedCarsArray = [...cars, newCar];
setCars(updatedCarsArray);Method 6
As @nilgun mentioned in the comment, you can use the react immutability helpers. I’ve found this to be super useful.
From the docs:
Simple push
var initialArray = [1, 2, 3];
var newArray = update(initialArray, {$push: [4]}); // => [1, 2, 3, 4]initialArray is still [1, 2, 3].
Method 7
Currently so many people facing problem to update the useState hook state. I use this approach to update it safely and wanted to share it here.
This is my state
const [state, setState] = useState([])
Suppose I have a object name obj1 and I want it to append in my state. I will suggest to do it like this
setState(prevState => [...prevState, obj1])
This will safely insert the object at the end and also keep the state consistency
Method 8
If you are using functional component please use this as below.
const [chatHistory, setChatHistory] = useState([]); // define the state
const chatHistoryList = [...chatHistory, {'from':'me', 'message':e.target.value}]; // new array need to update
setChatHistory(chatHistoryList); // update the stateMethod 9
For added new element into the array, push() should be the answer.
For remove element and update state of array, below code works for me. splice(index, 1) can not work.
const [arrayState, setArrayState] = React.useState<any[]>([]);
...
// index is the index for the element you want to remove
const newArrayState = arrayState.filter((value, theIndex) => {return index !== theIndex});
setArrayState(newArrayState);Method 10
Here’s a 2020, Reactjs Hook example that I thought could help others. I am using it to add new rows to a Reactjs table. Let me know if I could improve on something.
Adding a new element to a functional state component:
Define the state data:
const [data, setData] = useState([
{ id: 1, name: 'John', age: 16 },
{ id: 2, name: 'Jane', age: 22 },
{ id: 3, name: 'Josh', age: 21 }
]);
Have a button trigger a function to add a new element
<Button
// pass the current state data to the handleAdd function so we can append to it.
onClick={() => handleAdd(data)}>
Add a row
</Button>
function handleAdd(currentData) {
// return last data array element
let lastDataObject = currentTableData[currentTableData.length - 1]
// assign last elements ID to a variable.
let lastID = Object.values(lastDataObject)[0]
// build a new element with a new ID based off the last element in the array
let newDataElement = {
id: lastID + 1,
name: 'Jill',
age: 55,
}
// build a new state object
const newStateData = [...currentData, newDataElement ]
// update the state
setData(newStateData);
// print newly updated state
for (const element of newStateData) {
console.log('New Data: ' + Object.values(element).join(', '))
}
}
Method 11
Since you are not allowed to mutate the state directly, you cannot simply push an item to an array.
state = {
value: '',
list: ['a', 'b', 'c'],
};
this.setState({
list: [...this.state.list, newelement]
})
//or
onAddItem = () => {
// not allowed AND not working
this.setState(state => {
const list = state.list.push(state.value);
return {
list,
value: '',
};
});
};Method 12
this.setState(preState=>({arrayvar:[...prevState.arrayvar,newelement]}))this will work to solve this problem.
Method 13
What I do is update a value outside the state and do a forceupdate(), the less stuff managed by react the better since you have more control over what is updated.
Also creating a new array for every update may be too expensive if the updates are fast
Method 14
I am trying to push value in an array state and set value like this and define state array and push value by map function.
this.state = {
createJob: [],
totalAmount:Number=0
}
your_API_JSON_Array.map((_) => {
this.setState({totalAmount:this.state.totalAmount += _.your_API_JSON.price})
this.state.createJob.push({ id: _._id, price: _.your_API_JSON.price })
return this.setState({createJob: this.state.createJob})
})Method 15
I was having a similar issue when I wanted to modify the array state
while retaining the position of the element in the array
This is a function to toggle between like and unlike:
const liker = (index) =>
setData((prevState) => {
prevState[index].like = !prevState[index].like;
return [...prevState];
});as we can say the function takes the index of the element in the array state, and we go ahead and modify the old state and rebuild the state tree
Method 16
//get the value you want to add
const valor1 = event.target.elements.valor1.value;
//add in object
const todo = {
valor1,
}
//now you just push the new value into the state
//prevlista is the value of the old array before updating, it takes the old array value makes a copy and adds a new value
setValor(prevLista =>{
return prevLista.concat(todo) })
Method 17
This worked for me to add an array within an array
this.setState(prevState => ({
component: prevState.component.concat(new Array(['new', 'new']))
}));Method 18
//------------------code is return in typescript
const updateMyData1 = (rowIndex:any, columnId:any, value:any) => {
setItems(old => old.map((row, index) => {
if (index === rowIndex) {
return Object.assign(Object.assign({}, old[rowIndex]), { [columnId]: value });
}
return row;
}));Method 19
This code work for me:
fetch('http://localhost:8080')
.then(response => response.json())
.then(json => {
this.setState({mystate: this.state.mystate.push.apply(this.state.mystate, json)})
})All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0