Using a Set data structure in React's state

Solution 1:

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 */;
}