Create my Own React Signal

createEffect

let currentListener = undefined

const createEffect = (callback) => {
  currentListener = callback
  callback()
  currentListener = undefined
}

// Use case
createEffect(() => {
  console.log(someSignal())
})

createSignal

const createSignal = (initialValue) => {
  let value = initialValue
  
  // a set of callback functions, from createEffect
  const subscribers = new Set()
  
  const read = () => {
    if (currentListener !== undefined) {
      // before returning, track the current listener
      subscribers.add(currentListener)
    }
    
    return value
  }
  const write = (newValue) => {
    value = newValue
    
    // after setting the value, run any subscribers
    subscribers.forEach(subscriber => subscriber())
  }

  return [read, write]
}

// Usecase
const [count, setCount] = createSignal(0)