Reputation: 46547
I seem to be having issues pushing data into a state array. I am trying to achieve it this way:
this.setState({ myArray: this.state.myArray.push('new value') })
But I believe this is incorrect way and causes issues with mutability?
Upvotes: 305
Views: 704594
Reputation: 13747
If your array is too large for conventional [...oldArray, newValue]
to be feasible (as it is O(N)
complexity contrary to the O(1)
complexity of .push()
operation), you can wrap the array in an object to make sure to notify react of the update every time you mutate the array:
this.state = {
myArrayUpdateWrapper: {
myArray: [],
},
};
...
function pushToStateArray(newValue) {
const { myArray } = this.state.myArrayUpdateWrapper;
myArray.push(newValue);
this.setState({ myArrayUpdateWrapper: { myArray } });
}
I believe this approach is both compliant with React principles and is efficient too. Very surprised that nobody mentioned it so far, even in the downvoted answers.
Upvotes: 0
Reputation: 2809
this.state.myArray.push('new value')
returns the length of the extended array, instead of the array itself.Array.prototype.push().
I guess you expect the returned value to be the array.
It seems it's rather the behaviour of React:
NEVER mutate this.state directly, as calling setState() afterwards may replace the mutation you made. Treat this.state as if it were immutable.React.Component.
I guess, you would do it like this (not familiar with React):
this.setState({ myArray: [...this.state.myArray, 'new value'] })
Upvotes: 216
Reputation: 19
It is the best and simplest way in reactJS if you working in functional components!
setSelectedTaxDec((oldArray) => [...oldArray, dec]);
oldArray is the previousState in which I am pushing the new value dec, is the value I am pushing in this state array
you can avoid pushing the same item again by doing this
if (selectedTaxDec.includes(dec)) {
return;
} else {
setSelectedTaxDec((oldArray) => [...oldArray, dec]);
}
Upvotes: 0
Reputation: 911
If you use:
const[myArr, setMyArr] = useState([]);
for add:
setMyArr([...myArr, value]);
and for remove:
let index = myArr.indexOf(value);
if(index !== -1)
setPatch([...myArr.slice(0, index), ...myArr.slice(index, myArr.length-1)]);
Upvotes: 1
Reputation: 3750
const [array,setArray] = useState([]);
Push value at the end:
setArray(oldArray => [...oldArray,newValue] );
Push value at the start:
setArray(oldArray => [newValue,...oldArray] );
Upvotes: 243
Reputation: 57
setState([...prevState, {
label: newState.name,
value: newState.id
}]);
Was working with the dropdowns and wanted to implement this scenario there, i found this simple solution for dropdown with multiple values.
Upvotes: 3
Reputation: 375
I guess this is a little bit late for an answer but for those new to react
You can use this tiny package called immer
see this example: https://immerjs.github.io/immer/produce
Upvotes: -1
Reputation: 6693
Using react hooks, you can do following way
const [countryList, setCountries] = useState([]);
setCountries((countryList) => [
...countryList,
"India",
]);
Upvotes: 15
Reputation: 190
you are breaking React principles, you should clone the old state then merge it with the new data, you shouldn't manipulate your state directly, your code should go like this
fetch('http://localhost:8080').then(response => response.json()).then(json ={this.setState({mystate[...this.state.mystate, json]}) })
Upvotes: 0
Reputation: 765
Here you can not push the object to a state array like this. You can push like your way in normal array. Here you have to set the state,
this.setState({
myArray: [...this.state.myArray, 'new value']
})
Upvotes: 23
Reputation: 20168
In the following way we can check and update the objects
this.setState(prevState => ({
Chart: this.state.Chart.length !== 0 ? [...prevState.Chart,data[data.length - 1]] : data
}));
Upvotes: 2
Reputation: 3325
if u also want ur UI (ie. ur flatList) to be up to date, use PrevState: in the example below if user clicks on the button , it is going to add a new object to the list( both in the model and UI)
data: ['shopping','reading'] // declared in constructor
onPress={() => {this.setState((prevState, props) => {
return {data: [new obj].concat(prevState.data) };
})}}.
Upvotes: 2
Reputation: 34014
Never recommended to mutate the state directly.
The recommended approach in later React versions is to use an updater function when modifying states to prevent race conditions:
Push string to end of the array
this.setState(prevState => ({
myArray: [...prevState.myArray, "new value"]
}))
Push string to beginning of the array
this.setState(prevState => ({
myArray: ["new value", ...prevState.myArray]
}))
Push object to end of the array
this.setState(prevState => ({
myArray: [...prevState.myArray, {"name": "object"}]
}))
Push object to beginning of the array
this.setState(prevState => ({
myArray: [ {"name": "object"}, ...prevState.myArray]
}))
Upvotes: 165
Reputation: 147
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)})
})
Upvotes: 4
Reputation: 12384
Using es6 it can be done like this:
this.setState({ myArray: [...this.state.myArray, 'new value'] }) //simple value
this.setState({ myArray: [...this.state.myArray, ...[1,2,3] ] }) //another array
Upvotes: 316
Reputation: 968
You should not be operating the state at all. At least, not directly. If you want to update your array, you'll want to do something like this.
var newStateArray = this.state.myArray.slice();
newStateArray.push('new value');
this.setState(myArray: newStateArray);
Working on the state object directly is not desirable. You can also take a look at React's immutability helpers.
https://facebook.github.io/react/docs/update.html
Upvotes: 26
Reputation: 5316
You can use .concat
method to create copy of your array with new data:
this.setState({ myArray: this.state.myArray.concat('new value') })
But beware of special behaviour of .concat
method when passing arrays - [1, 2].concat(['foo', 3], 'bar')
will result in [1, 2, 'foo', 3, 'bar']
.
Upvotes: 17