Atoms in atom

atom() creates an atom config, which is an object, but it doesn't hold a value. Atom configs don't have string keys and we identify them with referential equality. In other words, we can use an atom config like a key.

Storing an atom config in useState

First things first, we can store an atom config in useState.

const Component = ({ atom1, atom2 }) => {
  const [selectedAtom, setSelectedAtom] = useState(atom1)
  const [value] = useAtom(selectedAtom)
  return (
    <div>
      Selected value: {value}
      <button onClick={() => setSelectedAtom(atom1)}>Select an atom</button>
      <button onClick={() => setSelectedAtom(atom2)}>Select another atom</button>
    </div>
  )
}

Note that we can pass atoms configs as props.

It might not make any sense, but we could create an atom config on demand.

const Component = () => {
  const [currentAtom, setCurrentAtom] = useState(() => atom(0))
  const [count, setCount] = useAtom(currentAtom)
  return (
    <div>
      Count: {count} <button onClick={() => setCount((c) => c + 1)}>+1</button>
      <button onClick={() => setCurrentAtom(atom(0))}>Create new</button>
    </div>
  )
}

Storing an atom config in atom

Likewise, we can store an atom confing as a value of another atom.

const firstNameAtom = atom('Tanjiro')
const lastNameAtom = atom('Kamado')

const showingNameAtom = atom(firstNameAtom)

const Component = () => {
  const [nameAtom, setNameAtom] = useAtom(showingNameAtom)
  const [name] = useAtom(nameAtom)
  return (
    <div>
      Name: {name}
      <button onClick={() => setNameAtom(firstNameAtom)}>Show First Name</button>
      <button onClick={() => setNameAtom(lastNameAtom)}>Show Last Name</button>
    </div>
  )
}

It's possible to create a derived atom.

const derivedNameAtom = atom(get) => {
  const nameAtom = get(showingNameAtom)
  return get(nameAtom)
})

// Or shoter version
const derivedNameAtom = atom(get) => get(get(showingNameAtom)))

To avoid confusing what is in atoms, naming atoms explicitly would be important. Also, TypeScript type information would be helpful.

Storing an array of atom configs in atom

Finally, the atoms in atom pattern is to store an array of atom config into an atom.

const countsAtom = atom([atom(1), atom(2), atom(3)])

const Counter = ({ countAtom }) => {
  const [count, setCount] = useAtom(countAtom)
  return (
    <div>
      {count} <button onClick={() => setCount((c) => c + 1)}>+1</button>
    </div>
  )
}

const Parent = () => {
  const [counts, setCounts] = useAtom(countsAtom)
  const addNewCount = () => {
    const newAtom = atom(0)
    setCounts((prev) => [...prev, newAtom])
  }
  return (
    <div>
      {counts.map((countAtom) => (
        <Counter countAtom={countAtom} key={countAtom} />
      ))}
      <button onClick={addNewCount}>Add</button>
    </div>
  )
}

The benefit of this approach is, if you increment a count, only the corresponding Counter component re-renders and no other components re-render.

It is important to note that anAtom.toString() return a unique id, which can be used for key in map.

Hint for TypeScript users

<Counter countAtom={countAtom} key={`${countAtom}`} />

Storing a map of atom configs in atom

Likewise, we can store an object map instead of an array.

const pricesAtom = atom({
  apple: atom(15),
  orange: atom(12),
  pineapple: atom(25),
})

const Fruit = ({ name, priceAtom }) => {
  const [price] = useAtom(priceAtom)
  return (
    <div>
      {name}: {price}
    </div>
  )
}

const Parent = () => {
  const [prices] = useAtom(pricesAtom)
  return (
    <div>
      {Object.keys(prices).map((name) => (
        <Fruit name={name} priceAtom={prices[name]} key={name} />
      ))}
    </div>
  )
}