CODEWITHSUNDEEP
reactjsreduxreducers
juanli
juanli

Reputation: 613

How does reducer works when state = initialState?

A sample code from an online course: 

import * as PlayerActionTypes from '../actiontypes/player';

const initialState = [
      {
        name: 'Jim Hoskins',
        score: 31,
      },
      {
        name: 'Andrew Chalkley',
        score: 20,
      },
      {
        name: 'Alena Holligan',
        score: 50,
      },
    ];

export default function Player(state=initialState, action) {
  switch(action.type) {
    case 'PlayerActionTypes.ADD_PLAYER':
      return [
        ...state,
        {
          name: action.name,
          score: 0,
        }
      ];

      case PlayerActionTypes.REMOVE_PLAYER:
        return [
          ...state.slice(0, action.index),
          ...state.slice(action.index+1),
        ]
  }
}

As the initialState is immutable(always the same), then the state is the same. So, let's say, if I first add an new player, a total of four players now. If then I want to remove the fourth player (whose index is 3 in the array), but how does it work? 

case PlayerActionTypes.REMOVE_PLAYER:
            return [
              ...state.slice(0, action.index),
              ...state.slice(action.index+1),
            ]

There is no index = 3 in the 'state' (only three players).

I don't understand. Please provide some help on my confusion. Thanks in advance.

Upvotes: 0

Views: 33

Answers (1)

tlt
tlt

Reputation: 15211

export default function Player(state=initialState, action) {

whenever action is dispatched, this function gets called with 2 parameters:

  1. state (which is current app state)
  2. action

based on those 2, this function returns new app state (which will be used as parameter in next call to this function with new action)

now, for the very first run, when app state is still null, the parameter state will be set to initialState. for every next run, since state will no longer be null, your state parameter will be whatever the current app state is.

check ES6 function default parameters.

Upvotes: 1

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