zustand/docs/guides/maps-and-sets-usage.md

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Map and Set Usage	11

You need to wrap Maps and Sets inside an object. When you want its update to be reflected (e.g. in React), you do it by calling setState on it:

You can view a codesandbox here: https://codesandbox.io/s/late-https-bxz9qy

import create from 'zustand'

const useFooBar = create(() => ({ foo: new Map(), bar: new Set() }))

function doSomething() {
  // doing something...

  // If you want to update some React component that uses `useFooBar`, you have to call setState
  // to let React know that an update happened.
  // Following React's best practices, you should create a new Map/Set when updating them:
  useFooBar.setState((prev) => ({
    foo: new Map(prev.foo).set('newKey', 'newValue'),
    bar: new Set(prev.bar).add('newKey'),
  }))
}