Version

3.0.11

Reproduction link

https://codesandbox.io/s/elegant-chandrasekhar-dlmc4?file=/src/main.js

Steps to reproduce

function createReactiveEffect<T = any>(
  fn: () => T,
  options: ReactiveEffectOptions
): ReactiveEffect<T> {
  const effect = function reactiveEffect(): unknown {
    if (!effect.active) {
      return options.scheduler ? undefined : fn() // reactiveEffect may return undefined
    }
    if (!effectStack.includes(effect)) {
      cleanup(effect)
      try {
        enableTracking()
        effectStack.push(effect)
        activeEffect = effect
        return fn()
      } finally {
        effectStack.pop()
        resetTracking()
        activeEffect = effectStack[effectStack.length - 1]
      }
    }
  } as ReactiveEffect
  effect.id = uid  
  effect.allowRecurse = !!options.allowRecurse
  effect._isEffect = true
  effect.active = true
  effect.raw = fn
  effect.deps = []
  effect.options = options
  return effect
}

for

const re = createReactiveEffect(() => 1)
re // => ReactiveEffect<number> ✅

but for

const re2 = createReactiveEffect(() => 1, {
  scheduler: setImmediate
})

What is expected?

re2 // => ReactiveEffect<number | undefined> 

What is actually happening?

re2 // => ReactiveEffect<number> 

Please correct me if I am wrong

Please double-check the reproduction you provided, it cannot reproduce the problem you described.

Please double-check the reproduction you provided, it cannot reproduce the problem you described.

@vue/reactivity does't export createReactiveEffect , but effect(exported) return result of createReactiveEffect finally, so they have same problem cause by createReactiveEffect. so Reproduction link just show example of effect

export function effect<T = any>(
  fn: () => T,
  options: ReactiveEffectOptions = EMPTY_OBJ
): ReactiveEffect<T> {
  if (isEffect(fn)) {
    fn = fn.raw
  }
  const effect = createReactiveEffect(fn, options)
  if (!options.lazy) {
    effect()
  }
  return effect
}

example for effect

const re2 = effect(() => 1, {
  scheduler: window.setImmediate
})

What is expected?

re2 // => ReactiveEffect<number | undefined> 

What is actually happening?

re2 // => ReactiveEffect<number> 

some type programing maybe helpful. I guess this need tests and .d.ts tests, which I don't know how to do it and can't send PR to fix it.

export function effect<T = any, O extends ReactiveEffectOptions = typeof EMPTY_OBJ>(
  fn: () => T,
  options: O = (EMPTY_OBJ as O)
): O extends { scheduler?: undefined } ? ReactiveEffect<T> : ReactiveEffect<T | undefined> {
  type Returned = O extends { scheduler?: undefined } ? ReactiveEffect<T> : ReactiveEffect<T | undefined>
  if (isEffect(fn)) {
    fn = fn.raw
  }
  const effect = createReactiveEffect(fn, options)
  if (!options.lazy) {
    effect()
  }
  return effect as Returned
}


const a = effect(() => 1) // => ReactiveEffect<number >

const a2 = effect(() => 1, { scheduler: setImmediate }) //  => ReactiveEffect<number | undefined>

I get your points now, but it will return undefined only if an effect has the scheduler option and it is inactive. But here is another issue discussing whether an effect should always return non-undefined when it is inactive, so we don't need to do anything for now.

doesn't have the problem after commit https://github.com/vuejs/vue-next/commit/03a7a73148a9e210a7889c7a2ecf925338735c70