JAVASCRIPT

redux reducer

const Reducer = (state=[],action) =>{
	switch(action.type){
      case'add':
        return [...state,action.payload]
      default:
        return state ;
    }
}

redux reducer

const reducer = (state = initialState, action) => {
  switch (action.type) {
    case "BUY_CAKE":
      return {
        numOfCakes: state.numOfCakes - 1,
      };
    default:
      return state;
  }
};

redux reducer example

import * as ActionTypes from './ActionTypes';

export const comments = (state = { errMess: null, comments: []}, action) => {
    switch (action.type) {
        case ActionTypes.ADD_COMMENTS:
            return {...state, errMess: null, comments: action.payload};

        case ActionTypes.COMMENTS_FAILED:
            return {...state, errMess: action.payload};

        default:
            return state;
    }
};

How reducer works in redux

const initialState = { value: 0 }

function counterReducer(state = initialState, action) {
  // Check to see if the reducer cares about this action
  if (action.type === 'counter/increment') {
    // If so, make a copy of `state`
    return {
      ...state,
      // and update the copy with the new value
      value: state.value + 1
    }
  }
  // otherwise return the existing state unchanged
  return state
}