Question

我从流程中得到一个错误，但是我正在检查以确保该错误永远不会发生。我怎样才能告诉流程一切正常？

/* @flow */

type A = {|
 test: string;
|}

type B = {|
 test: ?string;
|}

function foo(b: B): A {
  if (b && b.test) {
    return {
      test: b.test
    };
  }

  return { test: 'hi' };
}

const test: B = foo({ test: 'a' });

这是Flow给我的错误。

21: const test: B = foo({ test: 'a' });
                    ^ Cannot assign `foo(...)` to `test` because string [1] is incompatible with null or undefined [2] in property `test`.
References:
4:  test: string;
          ^ [1]
8:  test: ?string;
          ^ [2]

但是从代码中，我正在检查测试不能为null或未定义。因此，我不确定如何解决此问题。

Live Example Here

Answer 1

best_params_

这是因为/* @flow */ type A = {| test: string; |} type B = {| test: ?string; |} declare var TestA: A; declare var TestB: B; TestB = TestA; // ^ Cannot assign `TestA` to `TestB` because string [1] is incompatible with null or undefined [2] in property `test`不是{|test: string;}的子类型，所以我们可以更改{|test: ?string;}，但也可以更改TestB.test = null，其中TestA不应为空。

test是{|test: string;}的子类型。（{| +test: ?string;}-只读）

Answer 2

语法错误。我相信这是正确的代码。

/* @flow */

type A = {|
 test: string;
|}

type B = {|
 test: string;
|}

function foo(b: ?B): A {
  if (b && b.test) {
    return {
      test: b.test
    };
  }

  return { test: 'hi' };
}

const test: B = foo({ test: 'a' });

如何解决“因为字符串与null或未定义不兼容而无法分配”流错误？

2 个答案: