Updating the array object in React state using immutability helper

Im importing update from immutability helper here :)

this.state = {
  a: {
    b: [{ c: '', d: ''}, ...]
  }
} 


this.setState((prevState, props) => update(prevState, {
    a: {
        b: {
            $apply: b => b.map((item, ii) => {
                if(ii !== n) return item;
                return {
                    ...item,
                    c: 'new value'
                }
            })
        }
    }
}))

One way to do it would be using $set

let index = 0;
let newState = update(this.state, {
   a: {
     b: {
      [index]: {
               c: { $set: "new value"}
       }
    }
  }
});
this.setState(newState);

jsfiddle