如何编写断言 function 来断言可区分的联合类型？

Question

我有一个受歧视的工会：

type State =
    { tag: "A", /* other props */ }
  | { tag: "B", /* other props */ }

现在我有一段代码只能在某个state工作：

// Somewhere
const state: State = …;

// And later elsewhere
if (state.tag !== "A") {
  throw new AssertionError(…);
}

// Here I am sure `state` is of type A

我可以将条件重写为断言 function 吗？

// Somewhere
const assertState = (state: State, tag: State["tag"]) => ???

// And later:
assertState(state, "A");

// Here I am sure `state` is of type A

Answer 1

这可以使用断言函数来完成。

type State =
    { tag: "A", /* other props */ }
  | { tag: "B", /* other props */ }

function assertState (condition: boolean): asserts condition { 
  if (!condition) {
    throw new Error("message")
  }
}

function test(){
  let state: State = {} as any

  assertState(state.tag === 'A')

  state.tag // is only of type `A` now
}

操场