How to get back to the original state in redux?

I have to actions FETCH_REPOS and FILTER_BY_LANGUAGE. When the app runs, the repos are fetched from the server and loaded in the store.

Then I try to filter by language

case 'FILTER_BY_LANGUAGE':
  let newState =  { 
    repos:[...new Set(state.repos)]
         .filter(val => val.language == action.payload)
    }
    return newState

The scenario is, I fetch 30 repositories. Say 10 are JS, 10 PHP and 10 Node.

I then filter the JS repos. Now the store has 10 repos instead of 20, so when I try to filter again my PHP, those repos are go.

Show are refetch the data again or should I filter the data within the react props?

Here are the actions

export const fetchRepos = () => dispatch => {
    fetch('http://localhost:4000/originalrepos')
        .then(res => res.json())
        .then(repos => {
            let content = repos.map((data, i)=>{
                return {
                    title: data.name,
                    username: data.owner.login,
                    avatar: data.owner.avatar_url,
                    date: data.updated_at,
                }
            })
            dispatch({
                type: 'FETCH_REPOS',
                payload: content
            })
        })
}
export const filterByLanguage = language => dispatch => {
    dispatch({
        type: 'FILTER_BY_LANGUAGE',
        payload: language

    })
}

I fetchRepos on mount, then the FILTER_BY_LANGUAGE sort of reduces the store. An expected behaviour, so I'm wondering whether I need to run the same fetch within FILTER_BY_LANGUAGE or remove that action all together and take care of filtering the store at the props? Thought these are the reasons to use redux, right?

Solution

You can save your state whenever there's a state change with something like this:

var stateHistory = []

YourReducer = (state, action){

  //Whenever the reducer is called, it pushes to the history
  stateHistory.push(state)

  switch(action.type){
    ...
    case "UNDO":
      //fire the UNDO action to go back to the last recorded state
      return stateHistory.pop()
  }
}

You might want to check if the new state is the same as the last pushed state before adding it to the history object.