This doc describes `jotai/optics` bundle.


You have to install optics-ts to access this bundle and its functions.

npm install optics-ts
# or
yarn add optics-ts


focusAtom creates a new atom, based on the focus that you pass to it. This creates a derived atom that will focus on the specified part of the atom, and when the derived atom is updated, the derivee is notified of the update, and the equivalent update is done on the derivee.

See this:

const baseAtom = atom({ a: 5 }) // PrimitiveAtom<{a: number}>
const derivedAtom = focusAtom(baseAtom, (optic) => optic.prop('a')) // PrimitiveAtom<number>

So basically, we started with a PrimitiveAtom<{a: number}>, which has a getter and a setter, and then used focusAtom to zoom in on the a-property of the baseAtom, and got a PrimitiveAtom<number>. What is noteworthy here is that this derivedAtom is not only a getter, it is also a setter. If derivedAtom is updated, then equivalent update is done on the baseAtom.

The example below is simple, but it's a starting point. focusAtom supports many kinds of optics, including Lens, Prism, Isomorphism.

To see more advanced optics, please see the example at:


import { atom } from 'jotai'
import { focusAtom } from 'jotai/optics'

const objectAtom = atom({ a: 5, b: 10 })
const aAtom = focusAtom(objectAtom, (optic) => optic.prop('a'))
const bAtom = focusAtom(objectAtom, (optic) => optic.prop('b'))

const Controls = () => {
  const [a, setA] = useAtom(aAtom)
  const [b, setB] = useAtom(aAtom)
  return (
      <span>Value of a: {a}</span>
      <span>Value of b: {b}</span>
      <button onClick={() => setA((oldA) => oldA + 1)}>Increment a</button>
      <button onClick={() => setA((oldB) => oldB + 1)}>Increment b</button>