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👻 Primitive and flexible state management for React
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---title: Atoms in atomnav: 7.12---
`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.
```jsxconst 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.
```jsxconst 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 config as a value of another atom.
```jsxconst 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.
```jsconst derivedNameAtom = atom((get) => { const nameAtom = get(showingNameAtom) return get(nameAtom)})
// Or a shorter versionconst 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.
```jsxconst 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()` returns a unique id, which can be used as a `key` in a map.
### Hint for TypeScript users
```jsx<Counter countAtom={countAtom} key={`${countAtom}`} />```
## Storing a map of atom configs in atom
Likewise, we can store an object map instead of an array.
```jsxconst 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> )}```