proxyWithComputed

add a computation to a proxy

You can define own computed properties within a proxy. By combining with a memoization library such as proxy-memoize, optimizing function calls is possible.

Be careful not to overuse proxy-memoize because proxy-memoize and useSnapshot do similar optimization and double optimization may lead to less performance.

import { memoize } from 'proxy-memoize'
import { proxyWithComputed } from 'valtio/utils'

const state = proxyWithComputed(
  {
    count: 1,
  },
  {
    doubled: memoize((snap) => snap.count * 2),
  }
)

// Computed values accept custom setters too:
const state2 = proxyWithComputed(
  {
    firstName: 'Alec',
    lastName: 'Baldwin',
  },
  {
    fullName: {
      get: memoize((snap) => snap.firstName + ' ' + snap.lastName),
      set: (state, newValue) => {
        ;[state.firstName, state.lastName] = newValue.split(' ')
      },
    },
  }
)

// if you want a computed value to derive from another computed, you must declare the dependency first:
const state = proxyWithComputed(
  {
    count: 1,
  },
  {
    doubled: memoize((snap) => snap.count * 2),
    quadrupled: memoize((snap) => snap.doubled * 2),
  }
)

The last use case fails to infer types in TypeScript #192.