Correct way to push into state array
JavascriptReactjsImmutabilityJavascript Problem Overview
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?
Javascript Solutions
Solution 1 - Javascript
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
Solution 2 - Javascript
Array push returns length
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.
Immutability
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):
var joined = this.state.myArray.concat('new value');
this.setState({ myArray: joined })
Solution 3 - Javascript
> 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]
}))
Solution 4 - Javascript
Functional Components & React Hooks
const [array,setArray] = useState([]);
Push value at the end:
setArray(oldArray => [...oldArray,newValue] );
Push value at the start:
setArray(oldArray => [newValue,...oldArray] );
Solution 5 - Javascript
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.
Solution 6 - Javascript
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']
})
Solution 7 - Javascript
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']
.
Solution 8 - Javascript
Using react hooks, you can do following way
const [countryList, setCountries] = useState([]);
setCountries((countryList) => [ ...countryList, "India", ]);
Solution 9 - Javascript
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)})
})
Solution 10 - Javascript
React-Native
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) };
})}}.
Solution 11 - Javascript
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
}));
Solution 12 - Javascript
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]}) })
Solution 13 - Javascript
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.
Solution 14 - Javascript
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