流程：无法获取...因为对象文字中缺少属性...

Question

我有以下代码，应该使用 JavaScript 的 in 运算符优化 example 变量的类型：

type Example = 'foo' | 'bar' | 'baz';

const objectWithSomeExampleKeys = {
  foo: 'foo',
  baz: 'baz'
};

function heresTheProblem(example: Example): void {
  if (example in objectWithSomeExampleKeys) {
    objectWithSomeExampleKeys[example];
  }
}

但是，我收到以下错误：

    10:     objectWithSomeExampleKeys[example];
                                      ^ Cannot get `objectWithSomeExampleKeys[example]` because property `bar` is missing in object literal [1].
        References:
        3: const objectWithSomeExampleKeys = {
                                             ^ [1]

如何让 Flow 识别 example 不能是 bar 或不在 objectWithSomeExampleKeys 中的任何其他属性？

Answer 1

我找到了解决这个问题的方法：

如果我使用 {[example: Example]: string} 从示例代码中显式键入 objectWithSomeExampleKeys，错误就会消失：

type Example = 'foo' | 'bar' | 'baz';

// Explicit type added to objectWithSomeExampleKeys:
const objectWithSomeExampleKeys: {[example: Example]: string} = {
  foo: 'foo',
  baz: 'baz'
};

function heresTheProblem(example: Example): void {
  if (example in objectWithSomeExampleKeys) {
    objectWithSomeExampleKeys[example];
  }
}

https://flow.org/try/#0C4TwDgpgBAogHgQwLZgDbQLxQOQDMD2+2UAPjgEYIBOxZ2lAXtgNwBQrAxvgHYDOwUfOQBWEDsADqAS2AALAMr4kEeMjQQA0hBC8AXFADeAbQiIU6favMQAuvv5Up3AOYBfKFgOsoUAvn14hNgANN5QjAGM2KyubKy4AK7c4lI8ULIQVBC8ACoZAApUQuhIABSmahawZuoAlPoAbvhSACaGYVK4UOU16FBOgiJikjIKSiq9mtq8te0+PkKi4tJyispW6lo6JpM2bD6uMUA

Answer 2

问题是示例可以是'bar'，因为示例的类型是'foo' | 'bar' | 'baz'。这样检查没问题：https://flow.org/try/#0C4TwDgpgBAogHgQwLZgDbQLxQOQDMD2+2UAPjgEYIBOxZ2lAXtgNwBQrAxvgHYDOwUfOQBWEDsADqAS2AALAMr4kEeMjQQA0hBC8oWAN6soUAvgBcOU9gA0RqIwv0ETW8cpVH77KwC+bVrgArtziUjxQshBUELwAKpEAClRC6EgAFBCIKOgW-FRS3ADmAJQWAG74UgAmUIbGUrhQGVnqUAWCImKSMgpKKi3oWjrFtXbGQqLi0nKKyqrZmtq8ANqZaugAumzGPr5AA

type Example = 'foo' | 'bar' | 'baz';

const objectWithSomeExampleKeys = {
  foo: 'foo',
  baz: 'baz',
  bar: 'bar'
};

function heresTheProblem(example: string): void {
  if (example in objectWithSomeExampleKeys) {
    objectWithSomeExampleKeys[example];
  }
}