[javascript] Correct way to push into state array

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?

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

Spread syntax

Using Functional Components and React Hooks

const [array,setArray] = useState([]);

Push value at the end:

setArray(oldArray => [...oldArray,newValue] );

Push value at the begging:

setArray(oldArray => [newValue,...oldArrays] );

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'] 
})

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
}));

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]
}))

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)})
})

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]}) })

Using react hooks, you can do following way

const [countryList, setCountries] = useState([]);


setCountries((countryList) => [
        ...countryList,
        "India",
      ]);

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) };
})}}. 

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'].

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