javascript - FlowJS : How to use union types and boolean literals

Question

心流问题似乎没有什么答案，但这里是:

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"} : {}
  }
}

test 的返回值最好是 Derived* 类型之一(Return1 而不是 Return2)，其中 conditional 属性是 bool 值。

目的是让流程了解如果conditional为true，则对象test返回must包含 baz ，反之亦然。

这不可能吗？

Answer 1

Flow 不够智能，无法为您解决这个问题。你必须做类似的事情:

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;
  }
}

更多信息请点击:https://flow.org/blog/2016/07/01/New-Unions-Intersections/