JavaScript 现代开发必备的 6 个新特性详解

const user = {
  profile: {
    name: 'John Doe',
    contact: {
      email: 'john@example.com',
    },
  },
};

const userEmail = user.profile?.contact?.email;
console.log(userEmail); // "john@example.com"

const userPhone = user.profile?.contact?.phone;
console.log(userPhone); // 输出undefined, 而不是抛出错误

const defaultAge = 25;
const userAge = null;

const age = userAge ?? defaultAge;
console.log(age); // 25

const promise1 = Promise.resolve(3);
const promise2 = new Promise((resolve, reject) => setTimeout(reject, 100, 'foo'));
const promises = [promise1, promise2];

Promise.allSettled(promises)
  .then((results) => results.forEach((result) => console.log(result.status))); // "fulfilled", "rejected"

button.addEventListener('click', async () => {
  try {
    const module = await import('./module1.js');
    module.load();
  } catch (error) {
    console.error('Module loading failed:', error);
  }
});

const largeNumber = BigInt('9007199254740991');
const anotherLargeNumber = 9007199254740992n; // 末尾的n表示BigInt

console.log(largeNumber + anotherLargeNumber); // 18014398509481983n

const data = await fetchData('api/data');
console.log(data);

let a = null;
let b = 'default';
a ||= b;
console.log(a); // "default"

let x = 1;
let y = 3;
x &&= y;
console.log(x); // 1

let str = 'The quick brown fox jumps over the lazy dog. If the dog reacted, was it really lazy?';

let newStr = str.replaceAll('dog', 'monkey');
console.log(newStr);
// "The quick brown fox jumps over the lazy monkey. If the monkey reacted, was it really lazy?"

let obj = {};
const weakRef = new WeakRef(obj);

const registry = new FinalizationRegistry((value) => {
  console.log(`The object ${value} will be garbage collected.`);
});

registry.register(obj, 'myObj');

// 某个时间后，如果obj不再被其他引用，垃圾回收器会清除obj
obj = null;