React Redux Tutorials - 6 - Reducers

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redux テキストリンク

Reducer

  • specify how the app's state changes in response to actions sent to the store.
  • Reducer is a function that accepts state and actions as arguments, and returns the next state of the application.
  • (previousState, action) => newState 
    • // (1) define a default state
// state has to be represented by a single object.
const initialState = {
  numOfCakes: 10
}

// initialState is passed in as an argument to the reducer function

// (2) define a reducer function 
// (previousState, action) => newState
const reducer = (state = initialState, action) => {
  switch(action.type) {
    case BUY_CAKE: return {
      numOfCakes: state.numOfCakes - 1
    }
    // ! we are not mutating the state object but return a new object.

    // if there was an action which we haven't accounted for
    default: return state 
  }
}
    In reality, your state object might contain more than one property. That is why it is always safer to first create a copy of the state object and then change only the properties that need to.
    To make a copy of the state object, we use the spread operator (...state). We are asking the reducer to first make a copy of the state object and then only update the numOfCakes. If there were other properties they would remain unchanged.
    const reducer = (state = initialState, action) => {
  switch(action.type) {
    case BUY_CAKE: return {
      ...state, 
      numOfCakes: state.numOfCakes - 1
    }
    default: return state 
  }
}
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