es6如何使用非默认参数之前的默认参数？ [英] es6 how to use default parameters that go before non-default parameters?

问题描述

我的默认参数有点生锈，我想知道如果参数没有默认值，那么如何使用默认值？

I am a bit rusty on default parameters, and I am wondering how can I use a default value for a parameter if it goes before parameters without defaults?

下面的来自Redux.js 的示例，默认值 {} 为状态参数有用吗？ （因为你不能默认下一个参数）？

In the example from Redux.js below, when will the default value {} for the state parameter be useful? (since you can't default the next parameter)?

const todo = (state = {}, action) => {
  switch (action.type) {
    //...

    case 'TOGGLE_TODO':
      if (state.id !== action.id) {
        return state
      }

      return Object.assign({}, state, {
        completed: !state.completed
      })

    default:
      return state
  }
}

推荐答案

有关使用情况特定于 redux.js 。第一个参数的默认值在函数调用中通常是无用的，因为没有默认的第二个参数。

The usage in question is specific to redux.js. The default value for the first parameter is generally useless in function calls because of the second parameter without default.

但是，正如同一个教程前面的一段关于减速机：

However, as said earlier in the same tutorial about Reducers:

Redux将以第一个
时间的未定义状态调用我们的reducer。这是我们返回我们应用程序初始状态的机会：

Redux will call our reducer with an undefined state for the first time. This is our chance to return the initial state of our app:

function todoApp(state, action) {
  if (typeof state === 'undefined') {
    return initialState
  }
  //...
  return state
}

所以这里没有真正省略第一个参数。 Redux在初始化时提供 undefined 作为其值。只有在这种情况下，教程使用默认参数语法作为快捷方式：

So the 1st parameter isn't really omitted here. Redux is supplying undefined as its value on initialization. It is only in this case, the tutorial used default arguments syntax as a shortcut:

function todoApp(state = initialState, action) {
  //...
  return state
}

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