ECMAScript 2022 に入る予定の提案ら

ECMAScript 2022 に入ることが決まった(Stage 4 になった)プロポーザルについて雑にまとめていく。随時更新する。

Class Fields

class Counter {
  count = 0;
}

Sosuke Suzuki

Private Methods

class Counter {
  #count = 0;
  get #count() {
    return this.#count;
  }
  set #count(value) {
    this.#count = value;
  }
  #foo() {}
}

Sosuke Suzuki

Static Class Features

class Foo {
  static value = "foo";
}

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RegExp Match Indices

const re1 = /a+(?<Z>z)?/d;
const s1 = "xaaaz";
const m1 = re1.exec(s1);
m1.indices[0][0] === 1;
m1.indices[0][1] === 5;
s1.slice(...m1.indices[0]) === "aaaz";

Sosuke Suzuki

Top-level await

await somePromise;

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Ergonomic brand checks for private fields

class Foo {
  #brand;
  static isFoo(obj) {
    return #brand in obj;
  }
}

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Array.prototype.at

Propossal

[1, 2, 3, 4, 5].at(-1); // 5

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Object.hasOwn

Proposal

const foo = { bar: 3 };

Object.hasOwn(foo, "bar"); // true

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Class Static Blocks

Proposal

class Foo {
  static y;
  static {
    this.y = "y";
  }
}

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Error Cause

Proposal

async function doUploadJob() {
  await fetch("https://example.com/upload").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

Sosuke Suzuki

Arbitrary module namespace identifier names

これは TC39 プロセスにおけるプロポーザルではありませんが、構文の変更を伴う仕様の修正なのでここに記載します。

Pull Request

const foo = "foo";
export { foo as "😃 hey hey" };

import { "😃 hey hey" as foo } from "./module.js";
console.log(foo); // foo