Flow: Cannot get ... because property ... is missing in object literal

Question

I have the following code, which should refine the example variable's type using JavaScript's in operator:

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

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

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

But instead, I get the following error:

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

How do I get Flow to recognize that example cannot be bar or any other property not in objectWithSomeExampleKeys?

Answer 1

I found a solution to this problem:

If I explicitly type objectWithSomeExampleKeys from my sample code with {[example: Example]: string}, the error goes away:

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