Select

selectAtom

⚠️ Unlike its name, selectAtom is provided as an escape hatch. Using it means building not 100% pure atom model. Prefer using derived atoms and use selectAtom only when equalityFn or prevSlice is unavoidable.

Signatures

function selectAtom<Value, Slice>(
  anAtom: Atom<Value>,
  selector: (v: Value, prevSlice?: Slice) => Slice,
  equalityFn: (a: Slice, b: Slice) => boolean = Object.is,
): Atom<Slice>

This function creates a derived atom whose value is a function of the original atom’s value, determined by selector. The selector function runs whenever the original atom changes; it updates the derived atom only if equalityFn reports that the derived value has changed. By default, equalityFn is reference equality, but you can supply your favorite deep-equals function to stabilize the derived value where necessary.

Examples

const defaultPerson = {
  name: {
    first: 'Jane',
    last: 'Doe',
  },
  birth: {
    year: 2000,
    month: 'Jan',
    day: 1,
    time: {
      hour: 1,
      minute: 1,
    },
  },
}

// Original atom.
const personAtom = atom(defaultPerson)

// Tracks person.name. Updated when person.name object changes, even
// if neither name.first nor name.last actually change.
const nameAtom = selectAtom(personAtom, (person) => person.name)

// Tracks person.birth. Updated when year, month, day, hour, or minute changes.
// Use of deepEquals means that this atom doesn't update if birth field is
// replaced with a new object containing the same data. E.g., if person is re-read
// from a database.
const birthAtom = selectAtom(personAtom, (person) => person.birth, deepEquals)

Hold stable references

As always, to prevent an infinite loop when using useAtom in render cycle, you must provide useAtom a stable reference of your atoms. For selectAtom, we need both the base atom and the selector to be stable.

const [value] = useAtom(selectAtom(atom(0), (val) => val)) // So this will cause an infinite loop

You have multiple options in order to satisfy these constraints:

const baseAtom = atom(0) // Stable
const baseSelector = (v) => v // Stable
const Component = () => {
  // Solution 1: Memoize the whole resulting atom with "useMemo"
  const [value] = useAtom(useMemo(() => selectAtom(baseAtom, (v) => v), []))

  // Solution 2: Memoize the inline callback with "useCallback"
  const [value] = useAtom(
    selectAtom(
      baseAtom,
      useCallback((v) => v, []),
    ),
  )

  // Solution 3: All constraints are already satisfied
  const [value] = useAtom(selectAtom(baseAtom, baseSelector))
}

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