javascript - redux 中的命名变量过滤器语法

Question

我刚刚学习完 javascript 和 typescript(我知道 redux 不是 typescript)，但对 redux 过滤器的语法感到非常困惑:

function visibilityFilter(state = 'SHOW_ALL', action) {
   ...
}

我在javascript中似乎有默认变量，但只有当默认变量位于位置变量之后时。这里发生了什么？有人可以向我指出解释此语法的文档吗？

Answer 1

基本上，这意味着 reducer 期望接收一个操作，但如果第一个参数是未定义，则使用默认值。

function reducer(state = 'initial', action) {
  if (action.type === 'change') {
    return 'new';
  }
  
  return state;
}

console.log(
  'uses given initial state: ', 
  reducer('what', {})
);

console.log(
  'null is also considered a given state: ', 
  reducer(null, {})
);

console.log(
  'uses default state if first param is undefined: ',
  reducer(undefined, {})
); 

const myServerState = undefined;
console.log(
  'passing undefined might not be so obvious: ',
  reducer(myServerState, {})
);

// throws error because action is undefined
console.log(reducer({ type: 'change' }));