可以使用流迭代两个联合数组

Question

我有一个函数foo，它接受两个数组的联合，它唯一做的就是循环遍历数组，但是这样我得到了流类型错误，数组中缺少一些属性。难道不能这样做吗？我根本没有使用任何属性。它们都是数组，所以 flow 应该知道它们是可迭代的。

A live example in the flow editor

type Handle = {|
  +articleId: string,
  +type: 'handle',
  +accessories: Array<string>,
  +positionInfo: string,
|};

type Leg = {|
  +articleId: string,
  +type: 'leg',
|};

type Entity = Handle | Leg;

function foo(entities: Array<Handle> | Array<Leg>) {
  entities.forEach(() => {})
}

Answer 1

您可以键入包含Entity 对象的数组（例如Array<Entity>），您可以稍后对其进行细化。或者，您可以将输入键入为Array<Handle | Leg>，但由于您已经定义了Entity 类型，我们应该使用它。

(Try)

// I made these types writeable and non-exact since it
// doesn't seem to matter for the sake of this question.
// You can make them read-only and exact in your code if
// you want (and I personally would unless I had good
// reason not to).

type Handle = {
  type: 'handle',
  articleId: string,
  accessories: Array<string>,
  positionInfo: string,
}

type Leg = {
  type: 'leg',
  articleId: string,
}

type Entity = Handle | Leg;

function foo(entities: Array<Entity>) {
  entities.forEach(entity => {
    // At this point, flow knows that our `entity` variable
    // either contains a Handle or a Leg (from the above
    // definition of the `Entity` type.) We can use a
    // refinement to figure out which one it is:
    if (entity.type === 'leg') {
      const {type, articleId} = entity
      console.log("A leg", type, articleId) 
    } else if (entity.type === 'handle') {
      const {type, articleId, positionInfo} = entity
      console.log("A handle", type, articleId, positionInfo) 
    } else {
      // We can even assert that we covered all possible
      // cases by asserting that the `entity` value has
      // no remaining union cases and is therefore empty
      (entity: empty) 
    }
  })
}

const entitiesExample = [
  {articleId: 'blah', type: 'leg'},
  {articleId: 'toot', type: 'handle', accessories: [], positionInfo: 'bar'}
]

foo(entitiesExample)

旁注：如果我使用== 检查type 属性而不是上面的===，我注意到流在优化类型时遇到了麻烦。如果有人知道那里发生了什么，我很想知道，因为我的直觉是 == 应该可以正常工作。