ECMAScript 2022正式发布

• 引入了 top-level await，允许在模块的顶层使用关键字；

// awaiting.mjsimport { process } from "./some-module.mjs";const dynamic = import(computedModuleSpecifier);const data = fetch(url);export const output = process((await dynamic).default, await data);

// usage.mjsimport { output } from "./awaiting.mjs";export functionoutputPlusValue(value) { return output + value }console.log(outputPlusValue(100));setTimeout(() => console.log(outputPlusValue(100), 1000);

• 新的 class elements：公共和私有实例字段、公共和私有静态字段、私有实例方法和访问器以及私有静态方法和访问器；

• 类内的静态块，用于执行每个类的评估初始化；

• #x in obj语法，用于测试对象上是否存在私有字段；

classX{
  #foo;
  method() {
    console.log(this.#foo)
  }}

• 通过/d flag 的正则表达式匹配索引，为匹配的子字符串提供开始和结束索引；

const re1 = /a+(?<Z>z)?/d;// indices are relative to start of the input string:const s1 = "xaaaz";const m1 = re1.exec(s1);m1.indices[0][0] === 1;m1.indices[0][1] === 5;s1.slice(...m1.indices[0]) === "aaaz";m1.indices[1][0] === 4;m1.indices[1][1] === 5;s1.slice(...m1.indices[1]) === "z";m1.indices.groups["Z"][0] === 4;m1.indices.groups["Z"][1] === 5;s1.slice(...m1.indices.groups["Z"]) === "z";// capture groups that are not matched return `undefined`:const m2 = re1.exec("xaaay");m2.indices[1] === undefined;m2.indices.groups["Z"] === undefined;

• Error对象的cause属性，可用于记录错误的因果链；

async functiondoJob() {
  const rawResource = await fetch('//domain/resource-a')
    .catch(err => {
      throw new Error('Download raw resource failed', { cause: err });
    });
  const jobResult = doComputationalHeavyJob(rawResource);
  await fetch('//domain/upload', { method: 'POST', body: jobResult })
    .catch(err => {
      throw new Error('Upload job result failed', { cause: err });
    });}try {
  await doJob();} catch (e) {
  console.log(e);
  console.log('Caused by', e.cause);}// Error: Upload job result failed// Caused by TypeError: Failed to fetch

• Strings、Arrays 和 TypedArrays 的at方法，允许相对索引；

functionat(n) {
	// ToInteger() abstract op
	n = Math.trunc(n) || 0;
	// Allow negative indexing from the end
	if (n < 0) n += this.length;
	// OOB access is guaranteed to return undefined
	if (n < 0 || n >= this.length) return undefined;
	// Otherwise, this is just normal property access
	return this[n];}const TypedArray = Reflect.getPrototypeOf(Int8Array);for (const C of [Array, String, TypedArray]) {
    Object.defineProperty(C.prototype, "at",
                          { value: at,
                            writable: true,
                            enumerable: false,
                            configurable: true });}

• 以及Object.hasOwn，这是Object.prototype.hasOwnProperty的一个更简洁方便的替代方法。

let hasOwnProperty = Object.prototype.hasOwnPropertyif (hasOwnProperty.call(object, "foo")) {
  console.log("has property foo")}

if (Object.hasOwn(object, "foo")) {
  console.log("has property foo")}

具体可查看：

• https://262.ecma-international.org/13.0/index.html
• https://www.ecma-international.org/wp-content/uploads/ECMA-262_13th_edition_june_2022.pdf