Sometimes I want to know the previous value for an effect

effect-with-previous-value.ts

import { effect } from '@preact/signals-core'

export function effectWithPreviousValue<T>(
  initialValue: T,
  compute: (prev: T) => T
): () => void {
  let prev = initialValue

  return effect(() => {
    prev = compute(prev)
  })
}

import_map.json

{
  "imports": {
    "@preact/signals-core": "https://esm.sh/@preact/signals-core@1.2.3",
    "std/": "https://deno.land/std@0.182.0/"
  }
}

test.ts

import { effectWithPreviousValue } from './effect-with-previous-value.ts'
import { signal } from '@preact/signals-core'
import { assertEquals } from 'std/testing/asserts.ts'

Deno.test('receives the previous returned value for each effect run', () => {
  let counterIncreasedByMoreThanTwo = false
  let effectRunCount = 0
  const counter = signal(0)

  const dispose = effectWithPreviousValue<number>(0, prevNum => {
    effectRunCount += 1

    if (counter.value > prevNum + 2) {
      counterIncreasedByMoreThanTwo = true
    }

    return counter.value
  })

  try {
    assertEquals(effectRunCount, 1)
    assertEquals(counterIncreasedByMoreThanTwo, false)

    counter.value = 1
    assertEquals(effectRunCount, 2)
    assertEquals(counterIncreasedByMoreThanTwo, false)

    counter.value = 5
    assertEquals(effectRunCount, 3)
    assertEquals(counterIncreasedByMoreThanTwo, true)
  } finally {
    dispose()
  }
})