How to select a specific object from array inside setState?

Question

 class Example extends React.Component{ constructor(props){ super(props); this.state = { array: [{n: 0}, {n: 1}, {n: 2}] } } selectObjectFromArray = index => { this.setState({ array: //??? }) } 

We only know the index, where we want to edit the object in the array. We can't do like this.state.array[1] = ... , and we can't do setState({array[1]:... . I was considered about spread like: array: [...this.state.array, , but in this situation, we can't set, where we want to edit. So what can we do in this situation?

Answer 1

To update a state-array, you have to create a copy of it, update some entries and push the new array back to the state.

Here's a simple example where the button updates the last item:

import React from "react";
import ReactDOM from "react-dom";

class Example extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      array: [{ n: 0 }, { n: 1 }, { n: 2 }]
    }
  }

  render() {
    return <div>

    <ul> {
      this.state.array.map((item,key) => {
        return <li key={key} >{item.n} </li>
      })
    }
    </ul>


    <button onClick={this.updateLast}>update first</button>
    </div>
  }

  updateLast = () => {
    this.selectObjectFromArray(this.state.array.length -1 )
  }

  selectObjectFromArray = index => {
    // create a copy of the array        
    let newArr = this.state.array;

    // the item variable is optional but hope clarifies how this works
    let item = newArr[index];    
    item = {
      n: item.n * 2
    }

    newArr[index] = item;

    // update the state with the new data
    this.setState({
      array: newArr
      })
  }
}

const rootElement = document.getElementById("root");
ReactDOM.render(<Example />, rootElement);

This working example can also be found here: https://codesandbox.io/s/o4x6mpnozq

Answer 2

We only know the index, where we want to edit the object in the array

Given a knownIndex

this.setState({
  array:
    [ ...this.state.array.slice (0, knownIndex)  // <-- items up to the index
    , someUpdate(this.state.array[knownIndex]))  // <-- updated object
    , ...this.state.array.slice (knownIndex + 1) // <-- items after the index
    ]
})

Another way you could do it is using the Array .map function. Let's also make it a generic function

const updateAtIndex = (xs, i, f) =>
  xs .map ((x, j) => i === j ? f (x) : x)

In your component, you can use it like this

this.setState ({
  array: updateAtIndex (this.state.array, knownIndex, f)
})

Where f is some function to update the object, for example

// returns a new object with the n property doubled
const f = obj =>
  ({ n: obj.n * 2 })

---

When writing generic functions, I like to make them a little more robust. If you use this technique in your program, I would recommend a few changes to the functions above. These changes communicate the parameter types more effectively and allow the reader to better infer the functions return type.

const identity = x =>
  x

const updateAtIndex = (xs = [], i = 0, f = identity) =>
  xs .map ((x, j) => i === j ? f (x) : x)

const f = ({ n = 0 }) =>
  ({ n: n * 2 })