如何在 ES5 Redux reducer 中初始化默认数据？

Question

目前我还不能使用 ES6/ES2015，而且我坚持使用 ES5 来编写 Redux reducers。由于 reducer 的状态参数必须是不可变的，即使它是未定义的，我想出了以下模式：

function myState( state, action ) {
    if ( typeof state === 'undefined' ) {
        return myState( { value1: 'foo', value2: 'bar' }, action );
    }
    // actual state calculation here
}

关于如何使用 ES5 确保默认值的任何替代建议或意见？

编辑： 经过一些问题和建议：我进行递归调用的原因是我非常重视 "state is immutable"。因此，即使 state 参数为 undefined，我也不会更改参数变量本身。我是不是把不变性看得太过分了？

Answer 1

另一种选择是这样的：

var initialState = { value1: 'foo', value2: 'bar' };
function myState( state, action ) {
  state = state || initialState;
  // actual state calculation here
}

Answer 2

Redux 不会强制你使用默认的参数语法。它只关心当它给你 undefined 作为状态时，你 return 其他东西，以便你的应用程序能够使用初始状态树启动。

ES6 中的这个函数：

function counter(state = 0, action) {
  switch (action.type) {
  case 'INCREMENT':
    return state + 1
  case 'DECREMENT':
    return state + 1
  default:
    return state
  }
}

相当于ES5中的这个函数：

function counter(state, action) {
  if (state === undefined) {
    state = 0
  }

  switch (action.type) {
  case 'INCREMENT':
    return state + 1
  case 'DECREMENT':
    return state + 1
  default:
    return state
  }
}

验证这一点的一个好方法是run this code through the Babel REPL。

The reason I do a recursive call is that I take the "state is immutable" very seriously. So even when the state parameter is undefined, I don't change the parameter variable itself.

这里不需要递归调用。我认为您的问题可能包含一些关于 突变和引用分配之间差异的混淆。

写的时候

var x = { lol: true }
x.lol = false

你正在变异 x 对象。这是 Redux 所不允许的。

但是当你写

var x = { lol: true }
x = { lol: false }

原始对象保持完整。 x“绑定”（也称为“变量”）刚刚开始指向另一个对象。

Redux 不关心你是否更改了 state 参数所指的内容。它在您的函数中是本地的。不管你是否return，只要你不改变实际对象或它里面的任何对象.

，改变参考很好
仅仅改变变量所指的内容不会改变对象：

// good: local variable called "state" refers to a different number state = state + 1 // good: local variable called "state" refers to a different array state = state.concat([42]) // good: local variable called "state" refers to a different string state = state + ", lol"

然而，在对象本身或它链接到的对象内部改变某些东西 ，无论是否深入，都是突变，Redux 不允许：

// bad: object that local variable "state" refers to has been mutated state.counter = state.counter + 1 // bad: object that local variable "state" refers to has been mutated var sameObjectAsState = state state.counter = state.counter + 1 // bad: array that local variable "state" refers to has been mutated state.push(42) // bad: array that local variable "state" refers to has been mutated var sameArrayAsState = state sameArrayAsState.push(42) // bad: object that is linked from the object that local variable "state" refers to has been mutated state.something.deep.counter = 42 // bad: object that is linked from the object that local variable "state" refers to has been mutated var somethingDeep = state.something.deep somethingDeep.counter = 42

如何在 ES5 Redux reducer 中初始化默认数据？

How to initialize default data in ES5 Redux reducer?

javascript

ecmascript-5

redux