javascript - 流量: Cannot assign _ to _ because property _ is missing in _

Question

我在 try Flow 编辑器中重现了我的情况，可以访问 here .

以下是链接发生问题时的代码:

/* @flow */

type PayloadType = 1 | 2 | 3;

type Transaction = {
  amount: number,
  destination: string
}

function create(type: PayloadType, transaction: Transaction): void {
  transaction.amount = 10;
  transaction.destination = "8ca76aff-8fe8-4715-9e9a-2ad0630d45a0"

  if ((type: PayloadType) === 4) {
    transaction.message = "Hello";
  }
}

const transaction: Transaction = {}
create(1, transaction)

错误是:

Cannot assign "Hello!" to transaction.message because property message is missing in Transaction 1.

我分配无效属性的行永远不应该被执行。我假设 Flow 应该知道这一点，因为 type 永远不可能是 4，因此条件永远不会为 true。

编辑:这是一个link举一个更现实的例子。

Answer 1

我认为这里的问题不是出现错误，而是错误出现得太晚了，如果在 if 语句中删除 type 的转换，错误会更早出现，什么我相信这是更预期的结果。

/* @flow */

type PayloadType = 1 | 2 | 3;

type Transaction = {
  amount: number,
  destination: string
}

function create(type: PayloadType , transaction: Transaction): void {
  transaction.amount = 10;
  transaction.destination = "8ca76aff-8fe8-4715-9e9a-2ad0630d45a0"

  if (type === 4) {
    transaction.message = "Hello";
  }
}

const transaction: Transaction = {}
create(1, transaction)

Flow Try for change