ES2021更新内容

ES2021 功能

• 新增三个逻辑运算符

• 数字分隔符

• Promise.any & AggregateError

• String.prototype.replaceAll

• WeakRefs & FinalizationRegistry Objects

三个逻辑运算符

//"Or Or Equals"
x ||= y;
x || (x = y);

// "And And Equals"
x &&= y;
x && (x = y);

// "QQ Equals"
x ??= y;
x ?? (x = y);

数字分隔符

支持数字组直接的视觉分离

1_000_000_000           // Ah, so a billion
101_475_938.38          // And this is hundreds of millions

let fee = 123_00;       // $123 (12300 cents, apparently)
let fee = 12_300;       // $12,300 (woah, that fee!)
let amount = 12345_00;  // 12,345 (1234500 cents, apparently)
let amount = 123_4500;  // 123.45 (4-fixed financial)
let amount = 1_234_500; // 1,234,500

0.000_001 // 1 millionth
1e10_000  // 10^10000 -- granted, far less useful / in-range...
0xA0_B0_C0;

Promise.any & AggregateError

Promise.any([
  fetch('https://v8.dev/').then(() => 'home'),
  fetch('https://v8.dev/blog').then(() => 'blog'),
  fetch('https://v8.dev/docs').then(() => 'docs')
]).then((first) => {
  // 只要有一个状态为fulfilled就执行
  console.log(first);
  // → 'home'
}).catch((error) => {
  // 所有promises状态为rejected时执行
  console.log(error);
});

String.prototype.replaceAll

// String.prototype.replaceAll(searchValue, replaceValue)

'x'.replace('', '_');
// → '_x'

'xxx'.replace(/(?:)/g, '_');
// → '_x_x_x_'

'xxx'.replaceAll('', '_');
// → '_x_x_x_'

WeakRefs & FinalizationRegistry Objects

WeakRef 的一个实例创建了一个对给定对象的引用，如果该对象仍然在内存中，则返回该对象；如果目标对象已经被垃圾回收，则返回未定义的对象。

FinalizationRegistry 的实例在注册的目标对象被垃圾收集后触发回调函数。

let target = {};
let wr = new WeakRef(target);
// wr === target  false

// Creating a new registry
const registry = new FinalizationRegistry(heldValue => {
  // ....
});

registry.register(myObject, "some value", myObject);
// ...some time later, if you don't care about `myObject` anymore...
registry.unregister(myObject);