Dùng useReducer thay thế cho useState
Tags: react
Published: 2024-01-27 15:26:08

## Lý do


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/u6qabo9kfp1esar8usbg.png)
Source: https://www.frontendmag.com/tutorials/usereducer-vs-usestate/
## Cách bình thường, với useState

```
function useUndo(initialPresent) {
  const [state, setState] = React.useState({
    past: [],
    present: initialPresent,
    future: [],
  })

  const canUndo = state.past.length !== 0
  const canRedo = state.future.length !== 0

  const undo = React.useCallback(() => {
    setState(currentState => {
      const {past, present, future} = currentState

      if (past.length === 0) return currentState

      const previous = past[past.length - 1]
      const newPast = past.slice(0, past.length - 1)

      return {
        past: newPast,
        present: previous,
        future: [present, ...future],
      }
    })
  }, [])

  const redo = React.useCallback(() => {
    setState(currentState => {
      const {past, present, future} = currentState

      if (future.length === 0) return currentState

      const next = future[0]
      const newFuture = future.slice(1)

      return {
        past: [...past, present],
        present: next,
        future: newFuture,
      }
    })
  }, [])

  const set = React.useCallback(newPresent => {
    setState(currentState => {
      const {present, past} = currentState
      if (newPresent === present) return currentState

      return {
        past: [...past, present],
        present: newPresent,
        future: [],
      }
    })
  }, [])

  const reset = React.useCallback(newPresent => {
    setState(() => ({
      past: [],
      present: newPresent,
      future: [],
    }))
  }, [])

  return [state, {set, reset, undo, redo, canUndo, canRedo}]
}
```

## Refactor với useReducer 

```
const UNDO = 'UNDO'
const REDO = 'REDO'
const SET = 'SET'
const RESET = 'RESET'

function undoReducer(state, action) {
  const {past, present, future} = state
  const {type, newPresent} = action

  switch (type) {
    case UNDO: {
      if (past.length === 0) return state

      const previous = past[past.length - 1]
      const newPast = past.slice(0, past.length - 1)

      return {
        past: newPast,
        present: previous,
        future: [present, ...future],
      }
    }

    case REDO: {
      if (future.length === 0) return state

      const next = future[0]
      const newFuture = future.slice(1)

      return {
        past: [...past, present],
        present: next,
        future: newFuture,
      }
    }

    case SET: {
      if (newPresent === present) return state

      return {
        past: [...past, present],
        present: newPresent,
        future: [],
      }
    }

    case RESET: {
      return {
        past: [],
        present: newPresent,
        future: [],
      }
    }
  }
}

function useUndo(initialPresent) {
  const [state, dispatch] = React.useReducer(undoReducer, {
    past: [],
    present: initialPresent,
    future: [],
  })

  const canUndo = state.past.length !== 0
  const canRedo = state.future.length !== 0
  const undo = React.useCallback(() => dispatch({type: UNDO}), [])
  const redo = React.useCallback(() => dispatch({type: REDO}), [])
  const set = React.useCallback(
    newPresent => dispatch({type: SET, newPresent}),
    [],
  )
  const reset = React.useCallback(
    newPresent => dispatch({type: RESET, newPresent}),
    [],
  )

  return [state, {set, reset, undo, redo, canUndo, canRedo}]
}
```