TypeScript API Reflect.getMetadata 返回 undefined 的解决办法

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

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