What is the best way to add a value to an array in state

I have an array in state, let's say this.state.arr. I want to add something to this state property, and then change some more properties.

Option 1

onChange(event){
    this.state.arr.push('newvalue');
    ...
    this.setState({some:'val',arr:this.state.arr})
}

Option 2

onChange(event){
    var newArr = this.state.arr;
    ...
    newArr.push('newvalue');
    ...
    this.setState({some:'val',arr:newArr})
}

So.. I know this.state is supposed to be treated immutable. But is it ok to use it like in option 1 where I still set the state from it, or do I need to go with something like option 2, and thus always first making a copy in memory

For now, this is the best way.

this.setState(previousState => ({
    myArray: [...previousState.myArray, 'new value']
}));

Both of the options you provided are the same. Both of them will still point to the same object in memory and have the same array values. You should treat the state object as immutable as you said, however you need to re-create the array so its pointing to a new object, set the new item, then reset the state. Example:

onChange(event){
    var newArray = this.state.arr.slice();    
    newArray.push("new value");   
    this.setState({arr:newArray})
}

Another simple way using concat:

this.setState({
  arr: this.state.arr.concat('new value')
})

If you are using ES6 syntax you can use the spread operator to add new items to an existing array as a one liner.

// Append an array
const newArr = [1,2,3,4]
this.setState(prevState => ({
  arr: [...prevState.arr, ...newArr]
}));

// Append a single item
this.setState(prevState => ({
  arr: [...prevState.arr, 'new item']
}));

Short way with useState hook:

const [value, setValue] = useState([])
setValue([...value, newvalue])

the best away now.

this.setState({ myArr: [...this.state.myArr, new_value] })

onChange() {
     const { arr } = this.state;
     let tempArr = [...arr];
     tempArr.push('newvalue');
     this.setState({
       arr: tempArr
    });
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"></script>

handleValueChange = (value) => {
      let myArr= [...this.state.myArr]
      myArr.push(value)
      this.setState({
         myArr
     })

This might do the work.

For functional components with hooks

const [searches, setSearches] = useState([]);

// Using .concat(), no wrapper function (not recommended)
setSearches(searches.concat(query));

// Using .concat(), wrapper function (recommended)
setSearches(searches => searches.concat(query));

// Spread operator, no wrapper function (not recommended)
setSearches([...searches, query]);

// Spread operator, wrapper function (recommended)
setSearches(searches => [...searches, query]);

source: https://medium.com/javascript-in-plain-english/how-to-add-to-an-array-in-react-state-3d08ddb2e1dc

React hook - useState (2022)

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

const handleChange = (newValue) => {
  setArray((array) => [...array, newValue]);
};

If you want to keep adding a new object to the array i've been using:

_methodName = (para1, para2) => {
  this.setState({
    arr: this.state.arr.concat({para1, para2})
  })
}

This might not directly answer your question but for the sake of those that come with states like the below

 state = {
    currentstate:[
     {
    id: 1 ,
    firstname: 'zinani',
    sex: 'male'

     }
    ]
    
 }

Solution

const new_value = {
    id: 2 ,
    firstname: 'san',
    sex: 'male'

     }

Replace the current state with the new value

 this.setState({ currentState: [...this.state.currentState, new_array] })