How to Update `state.item[1]` with `setState` in React?

How to Update State.item[1] Using setState

In a React application, updating the state of an array or object can be tricky. This article will walk you through如何更新 state.item[1] in the state using setState, a common task in React.

Problem

Within the provided React component, the goal is to create a dynamic form where users can design their own fields. The state initially looks like this:

this.state.items[1] = {
  name: 'field 1',
  populate_at: 'web_start',
  same_as: 'customer_name',
  autocomplete_from: 'customer_name',
  title: ''
};

The issue arises when trying to update the state when a user changes any of the values. Targeting the correct object becomes difficult.

Solution

To update the state correctly, follow these steps:

1. Make a shallow copy of the items.
2. Make a shallow copy of the item you want to mutate.
3. Replace the property you're interested in.
4. Put it back into your array. Note that you are mutating the array here, which is why making a copy first is crucial.
5. Set the state to your new copy.

Example Implementation:

handleChange: function (e) {
  // 1. Make a shallow copy of the items
  let items = [...this.state.items];
  // 2. Make a shallow copy of the item you want to mutate
  let item = {...items[1]};
  // 3. Replace the property you're intested in
  item.name = 'newName';
  // 4. Put it back into our array. N.B. we *are* mutating the array here, 
  //    but that's why we made a copy first
  items[1] = item;
  // 5. Set the state to our new copy
  this.setState({items});
}

Alternative Implementations:

• Combining steps 2 and 3:

let item = {...items[1], name: 'newName'}

• Single-line implementation:

this.setState(({items}) => ({
  items: [
    ...items.slice(0, 1),
    {...items[1], name: 'newName'},
    ...items.slice(2)
  ]
}));

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