TypeScript API Reflect.getMetada

我有下面这段测试代码：

function validate(
  target: Object,
  key: string,
  descriptor: PropertyDescriptor
) {
  const originalFn = descriptor.value;

  // 获取参数的编译期类型
  const designParamTypes = Reflect
    .getMetadata('design:paramtypes', target, key);

  descriptor.value = function (...args: any[]) {
    args.forEach((arg, index) => {

      const paramType = designParamTypes[index];

      const result = arg.constructor === paramType
        || arg instanceof paramType;

      if (!result) {
        throw new Error(
          `Failed for validating parameter: ${arg} of the index: ${index}`
        );
      }
    });

    return originalFn.call(this, ...args);
  }
}

class C {
  @validate
  sayRepeat(word: string, x: number) {
    return Array(x).fill(word).join('');
  }
}

const c = new C();
console.log('Ethan');
c.sayRepeat('hello', 2); // pass
c.sayRepeat('', 'lol' as any); // throw an error

执行时，发现第 17 行的代码，Reflect.getMetadata 返回 undefined：

经过研究，发现了这个 StackOverflow 帖子：

https://stackoverflow.com/questions/65246467/reflection-metadata-designparamtypes-returning-undefined-on-deno

需要修改 tsconfig.json 的内容：

添加这两行：

  "experimentalDecorators": true,
    "emitDecoratorMetadata": true,

之后问题解决：

更多Jerry的原创文章，尽在："汪子熙":