ECMAScript 2023 的最终版本预计将于今年 6 月底发布。最新举行的一次 TC39 会议基本已经确定 了 ECMAScript 2023 的新功能列表，预计不会再有任何重大的编辑更改。
本文整理了在 2023 年进入 Stage 4 的几个提案。按照 TC39 的运作流程，每个提案都从 Stage 0 开始，进入 Stage 4 则意味 着该提案已被 ECMAScript 编辑签署同意意见，成为了事实上的标准特性。
Array find from last
该提案在Array和TypedArray原型上增加了findLast()和findLastIndex() 方法。它们与find()和findIndex()做同样的事情，但顺序相反。这两个方法都很方便，可跳过创建临时的重复、突变和混乱的索引减法。
const isEven = (number) => number % 2 === 0;
const numbers = [1, 2, 3, 4];
// from first to the last lookup
console.log(numbers.find(isEven));
// 2
console.log(numbers.findIndex(isEven));
// 1
// from last to the first lookup
console.log(numbers.findLast(isEven));
// 4
console.log(numbers.findLastIndex(isEven));
// 3
Hashbang Grammar
Hashbang，也称为 shebang，是可执行脚本开头的字符序列，用于定义要运行的程序的解释器。当 Unix 内核的程序加载器执行 JavaScript 程序时，主机会剥离 hashbang 以生成有效源，然后再将其传递给引擎。Hashbang Grammar 提案标准化了它的完成方式。
#!/usr/bin/env node
console.log('hi 👋');
Symbols as WeakMap keys
在 JavaScript 中，Objects 和 Symbols 被保证是唯一并且不能被重新创建的，这使得它们都是WeakMapkeys 的理想候选者。以前的版本或规范只允许以这种方式使用 Objects ，但新的 Symbols as WeakMap keys 提案则提出将 non-registered Symbols 添加到允许的键列表中。
const weak = new WeakMap();
const key = Symbol("ref");
weak.set(key, "ECMAScript 2023");
console.log(weak.get(key));
// ECMAScript 2023
Change Array by Copy
Array.prototype上的reverse()、sort()和splice()方法就地改变数组。Change Array by Copy 提案添加了那些返回新 copy 方法的等价物—— toReversed()、toSorted()和toSpliced()。该提案还添加了一个with()方法，该方法返回一个新的数组，其中给定索引处的元素被替换为给定值，以避免使用 bracket notation 的就地突变。
const original = [1, 2, 3, 4];
const reversed = original.toReversed();
console.log(original);
// [ 1, 2, 3, 4 ]
console.log(reversed);
// [ 4, 3, 2, 1 ]
const original = [1, 3, 2, 4];
const sorted = original.toSorted();
console.log(original);
// [ 1, 3, 2, 4 ]
console.log(sorted);
// [ 1, 2, 3, 4 ]
const original = [1, 4];
const spliced = original.toSpliced(1, 0, 2, 3);
console.log(original);
// [ 1, 4 ]
console.log(spliced);
// [ 1, 2, 3, 4 ]
const original = [1, 2, 2, 4];
const withThree = original.with(2, 3);
console.log(original);
// [ 1, 2, 2, 4 ]
console.log(withThree);
// [ 1, 2, 3, 4 ]
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