FlowJS：如何使用联合类型和布尔文字

Question

Flow questions seem to garner few answers, but here goes:

type Base = {
  foo: string,
  bar: string
}
type Derived1 = Base & {
  conditional: false
}
type Derived2 = Base & {
  conditional: true,
  baz: string
}

type One = {
  foo: string,
  bar: string,
  conditional: boolean
}
type Two = One & {
  baz: string
}

type Return1 = Derived1 | Derived2  // fails

type Return2 = One | Two  // works, but not desired

function test(conditional: boolean): Return1 {
  return {
    foo: "foo",
    bar: "bar",
    conditional,
    ...conditional ? {baz: "baz"} : {}
  }
}

It is preferable for the return value of test to be one of the Derived* types ( Return1 rather than Return2 ), where the conditional property is a boolean literal.

The intention is for flow to understand that if conditional is true , then the object test returns must contain baz and vice versa.

Is this not possible?

Answer 1

Flow isn't quite smart enough to figure it out for you. You have to do something like:

function test(conditional: boolean): Return1 {

  const base = {
    foo: "foo",
    bar: "bar",
  }

  if (!conditional) {
    return Object.assign({}, base, { conditional });
  } else {
    const result: Derived2 = Object.assign({}, base, { conditional }, {baz: "baz"});
    return result;
  }
}

More info here: https://flow.org/blog/2016/07/01/New-Unions-Intersections/