ECMAScript 2022 正式发布

ECMAScript 2022 现已获得 ECMA International 的批准。ECMAScript 是标准化的 JavaScript 语言，于 1997 年发布了第一版，现已发展成为世界上使用最广泛的通用编程语言之一。

本 Ecma 标准定义了 ECMAScript 2022 Language，是 ECMAScript 语言规范的第 13 版。

ECMAScript 2022 主要包含内容有：

• 引入了 top-level await，允许在模块的顶层使用关键字；

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

// usage.mjs
import { output } from "./awaiting.mjs";
export function outputPlusValue(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；

class X {
#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 function doJob() {
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；

function at(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.hasOwnProperty

if (hasOwnProperty.call(object, "foo")) {
console.log("has property foo")
}

简化为： 

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