Merge, Dedupe, and Update Arrays in a Redux Reducer

projectReducer.js

const ADD_PROJECTS = "projects/add_projects"

const initialState = {
 projects: []
}

function reducer(state = initialState, action = {}){
  switch(action.type){
    case ADD_PROJECTS:
      
      // Add a timestamp for the new projects
      const timeStamp = new Date().getTime()
      const stampedActionProjects = action.projects.map((proj) => {
        proj.timeStamp = timeStamp
        return proj
      })
      
      // combine the old projects with the new timeStamped ones
      // then sort by ID value
      const combinedArrays = [...state.projects, ...stampedActionProjects].sort((a, b) => {
        return a.id > b.id
      })
      
      const finalArray = combinedArrays.reduce((prev, cur, index, array) => {
       
        // This is the value that we'll return at the end of each reduce iteration
        let toReturn
        
        // Get a reference to the last item in the PREV array
        const lastObj = prev[prev.length - 1]
      

        // if the IDs are different, concat the cur obj into the prev array
        if(lastObj.id !== cur.id){
          toReturn = prev.concat(cur)
        } 

        // if the IDS are different, compare the timestamp values
        // store the most recent timestamp in the PREV array
        else if (lastObj.timeStamp < cur.timeStamp){
          prev.splice((prev.length - 1), 1, cur)
          toReturn = prev
        }

        // otherwise just return the previous value
        else {
         toReturn = prev   
        }

        return toReturn
      }, [combinedArrays[0]])
      
      
      return {
        projects: finalArray
      }

    default:
      return state
  }
}

function addProjects(projects){
  return {
    type: ADD_PROJECTS,
    projects: projects
  }
}