javascript如果声明问题

var x = 10;
let y = 20;
const z = 30;

function add(a, b) {
  return a + b;
}

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }

  sayHello() {
    console.log(`Hello, my name is ${this.name} and I am ${this.age} years old.`);
  }
}

// module1.js
export function add(a, b) {
  return a + b;
}

// module2.js
import { add } from './module1.js';

console.log(add(1, 2)); // 输出 3