How do I add an element to array in reducer of React native redux?

How do I add elements in my array arr[] of redux state in reducer? I am doing this-

import {ADD_ITEM} from '../Actions/UserActions'
const initialUserState = {
    arr:[]
}

export default function userState(state = initialUserState, action)
{
    console.log(arr);
    switch (action.type)
    {
        case ADD_ITEM: 
            return { 
                      ...state,
                      arr: state.arr.push([action.newItem])
                   }

        default:
            return state
    }
}

Two different options to add item to an array without mutation

case ADD_ITEM :
    return { 
        ...state,
        arr: [...state.arr, action.newItem]
    }

OR

case ADD_ITEM :
    return { 
        ...state,
        arr: state.arr.concat(action.newItem)
    }

push does not return the array, but the length of it (docs), so what you are doing is replacing the array with its length, losing the only reference to it that you had. Try this:

import {ADD_ITEM} from '../Actions/UserActions'
const initialUserState = {

    arr:[]
}

export default function userState(state = initialUserState, action){
     console.log(arr);
     switch (action.type){
        case ADD_ITEM :
          return { 
             ...state,
             arr:[...state.arr, action.newItem]
        }

        default:return state
     }
}

If you need to insert into a specific position in the array, you can do this:

case ADD_ITEM :
    return { 
        ...state,
        arr: [
            ...state.arr.slice(0, action.pos),
            action.newItem,
            ...state.arr.slice(action.pos),
        ],
    }