Using a Set data structure in React's state

Is it possible to use ES6's Set data structure in React?

For example, if I have a checklist composed of distinct items, and I'd like to maintain each item's checked state. I'd like to write something like this:

export default class Checklist extends React.Component {
  constructor(props) {
    super(props);
    
    this.state = {
      checkedItems: new Set()
    }
  }
  
  addItem(item) {
    //...?
  }
  
  removeItem(item) {
    //...?
  }
  
  getItemCheckedStatus(item) {
    return this.state.checkedItems.has(item);
  }

  // More code...
}

I understand there may be a problem with the fact that a Set is mutable by nature, and React performs a shallow comparison when updating the component, so it expects immutable objects to be passed and held in the state. However, is there a way to hold and maintain a Set object in the state?

Since react will identify state changes only if the state property was replaced, and not mutated (shallow compare), you'll have to create a new Set from the old one, and apply the changes to it.

This is possible since new Set(oldSet) !== oldSet.

const oldSet = new Set([1, 2]);
const newSet = new Set(oldSet);

console.log(oldSet === newSet);

---

How you use a Set in a class component:

export default class Checklist extends React.Component {
  constructor(props) {
    super(props);

    this.state = {
      checkedItems: new Set()
    }
    
    this.addItem = this.addItem.bind(this);
    this.removeItem = this.removeItem.bind(this);
  }

  addItem(item) {
    this.setState(({ checkedItems }) => ({
      checkedItems: new Set(checkedItems).add(item)
    }));
  }

  removeItem(item) {
    this.setState(({ checkedItems }) => {
      const newChecked = new Set(checkedItems);
      newChecked.delete(item);
      
      return {
       checkedItems: newChecked
      };
    });
  }

  getItemCheckedStatus(item) {
    return this.state.checkedItems.has(item);
  }

  // More code...
}

---

How to use a set with the useState() hook:

const Comp = () => {
  [state, setState] = useState(() => new Set());

  const addItem = item => {
    setState(prev => new Set(prev).add(item));
  }

  const removeItem = item => {
    setState(prev => {
      const next = new Set(prev);

      next.delete(item);

      return next;
    });
  }

  return /* JSX */;
}

state is just a common object, you can hold what ever you want in it. You can overwrite the shouldComponentUpdate lifecycle method to implement it yourself. You can use you own logic to figure out if the component needs to update or not. You don't even need to nest your set, you can just set it directly as the state:

this.state = new Set();