How do I update states `onChange` in an array of object in React Hooks

Question

I have retrieved datas stored using useState in an array of object, the datas was then outputted into form fields. And now I want to be able to update the fields (state) as I type.

I have seen examples on people updating the state for property in array, but never for state in an array of object, so I don't know how to do it. I've got the index of the object passed to the callback function but I didn't know how to update the state using it.

// sample data structure
const datas = [
  {
    id: 1,
    name: 'john',
    gender: 'm'
  }
  {
    id: 2,
    name: 'mary',
    gender: 'f'
  }
]

const [datas, setDatas] = useState([]);

const updateFieldChanged = index => e => {
  console.log('index: ' + index);
  console.log('property name: '+ e.target.name);

  setData() // ??
}

return (
  <React.Fragment>
    {datas.map((data, index) => {
      <li key={data.name}>
        <input type="text" name="name" value={data.name} onChange={updateFieldChanged(index)} />
      </li>
    })}
  </React.Fragment>
)

Answer 1

Here is how you do it:

// sample data structure
/* const data = [
  {
    id:   1,
    name: 'john',
    gender: 'm'
  }
  {
    id:   2,
    name: 'mary',
    gender: 'f'
  }
] */ // make sure to set the default value in the useState call (I already fixed it)

const [data, setData] = useState([
  {
    id:   1,
    name: 'john',
    gender: 'm'
  }
  {
    id:   2,
    name: 'mary',
    gender: 'f'
  }
]);

const updateFieldChanged = index => e => {
  console.log('index: ' + index);
  console.log('property name: '+ e.target.name);
  let newArr = [...data]; // copying the old datas array
  // a deep copy is not needed as we are overriding the whole object below, and not setting a property of it. this does not mutate the state.
  newArr[index] = e.target.value; // replace e.target.value with whatever you want to change it to

  setData(newArr);
}

return (
  <React.Fragment>
    {data.map((datum, index) => {
      <li key={datum.name}>
        <input type="text" name="name" value={datum.name} onChange={updateFieldChanged(index)}  />
      </li>
    })}
  </React.Fragment>
)

Answer 2

The accepted answer leads the developer into significant risk that they will mutate the source sequence, as witnessed in comments:

let newArr = [...data];
// oops! newArr[index] is in both newArr and data
// this might cause nasty bugs in React.
newArr[index][propertyName] = e.target.value; 

This will mean that, in some cases, React does not pick up and render the changes.

The idiomatic way of doing this is by mapping your old array into a new one, swapping what you want to change for an updated item along the way.

setDatas(
    datas.map(item => 
        item.id === index 
        ? {...item, someProp : "changed"} 
        : item 
))