W3Schools Tryit Editor

// Minimal local metadata helpers (no global mutations)
const __metaStore: WeakMap<object, Map<string | symbol, any[]>> = new WeakMap();
function __defineMetadata(key: string, value: any, target: object, propertyKey: string | symbol) {
  let targetMap = __metaStore.get(target);
  if (!targetMap) { targetMap = new Map(); __metaStore.set(target, targetMap); }
  const arr = targetMap.get(propertyKey) || [];
  const existingIndex = arr.findIndex((e: any) => e && e.__k === key);
  const item = { __k: key, __v: value };
  if (existingIndex >= 0) { arr[existingIndex] = item; } else { arr.push(item); }
  targetMap.set(propertyKey, arr);
}
function __getOwnMetadata(key: string, target: object, propertyKey: string | symbol) {
  const targetMap = __metaStore.get(target);
  if (!targetMap) return undefined;
  const arr = targetMap.get(propertyKey) || [];
  const found = arr.find((e: any) => e && e.__k === key);
  return found ? found.__v : undefined;
}

function validate(target: any, propertyKey: string, descriptor: PropertyDescriptor) {
  const originalMethod = descriptor.value;
  descriptor.value = function (...args: any[]) {
    const validations: Array<{index: number, type: string}> =
      __getOwnMetadata('validations', target, propertyKey) || [];

for (const validation of validations) {
      const { index, type } = validation;
      const param = args[index];
      let isValid = false;
      switch (type) {
        case 'string': isValid = typeof param === 'string' && param.length > 0; break;
        case 'number': isValid = typeof param === 'number' && !isNaN(param as any); break;
        case 'boolean': isValid = typeof param === 'boolean'; break;
      }
      if (!isValid) {
        throw new Error(`Parameter at index ${index} failed ${type} validation`);
      }
    }
    return originalMethod.apply(this, args);
  };
  return descriptor;
}

class UserService {
  @validate
  createUser(
    @validateParam('string') name: string,
    @validateParam('number') age: number,
    @validateParam('boolean') isActive: boolean
  ) {
    console.log(`Creating user: ${name}, ${age}, ${isActive}`);
  }
}

const service = new UserService();
service.createUser('John', 30, true);