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未知服务器上的Typescript属性类型保护

typeguards typescript

Vinz243 · 技术社区 · 3 年前

我正在尝试键入未知类型的守卫

const foo  = (obj: unknown) => {
    if (typeof obj === 'object' && obj) {
        if ('foo' in obj && typeof obj.foo === 'string') {
            return obj.foo;
        }
    }
};

But I'm getting

我还尝试了is表达式不起作用:

const foo  = (obj: unknown): obj is { foo: 'string' } => {
    if (typeof obj === 'object' && obj) {
        if ('foo' in obj && typeof obj.foo === 'string') {
            return obj;
        }
    }
    throw new Error();
};

2 回复 | 直到 3 年前

Cerbrus 3 年前

type fooObj = object & { foo: unknown };
const foo = (obj: unknown) => {
    if (typeof obj === 'object' && obj) {
        if ('foo' in obj && typeof (obj as fooObj).foo === 'string') {
            return (obj as fooObj).foo;
        }
    }
};

captain-yossarian from Ukraine 3 年前

请考虑使用此助手:

const hasProperty = <Obj, Prop extends string>(obj: Obj, prop: Prop)
  : obj is Obj & Record<Prop, unknown> =>
  Object.prototype.hasOwnProperty.call(obj, prop);

就你而言。 in here here 和 here

工作解决方案:

const hasProperty = <Obj, Prop extends string>(obj: Obj, prop: Prop)
  : obj is Obj & Record<Prop, unknown> =>
  Object.prototype.hasOwnProperty.call(obj, prop);

const foo = (obj: unknown) => {
  if (typeof obj === 'object' && obj) {
    if (hasProperty(obj, 'foo') && typeof obj.foo === 'string') {
      return obj.foo;
    }
  }
};

Playground

obj assert function :

const hasProperty = <Obj, Prop extends string>(obj: Obj, prop: Prop)
  : obj is Obj & Record<Prop, unknown> =>
  Object.prototype.hasOwnProperty.call(obj, prop);

function foo(obj: unknown): asserts obj is { foo: string } {
  const isValid =
    typeof obj === 'object' &&
    obj &&
    hasProperty(obj, 'foo') &&
    typeof obj.foo === 'string';

  if (!isValid) {
    throw new Error();
  }

};

declare var obj: unknown;

foo(obj);

obj.foo // ok

Playground