ECMAScript 2023 is here! 🚀 These are the features that will become part of the JavaScript programming language:

1. Change Array by copy
2. Array find from last index
3. Symbols as WeakMap keys
4. Hashbang Grammar (#!)

Change Array by copy

Gives new methods to Array.prototype and TypedArray.prototype to perform certain actions in a new copy of the array, that means without mutating the original array.

The reverse(), sort() and splice() methods mutate the array, so using the new toReversed(), toSorted() and toSpliced() you can reverse, sort and splice without mutating the target array.

Another method was added: with(index, value) that returns a new array with the element at the given index replaced with the value provided.

Array.toReversed()

const original = [1, 2, 3, 4, 5];
const newReversed = original.toReversed(); // Doesn't change the original array

original; // [1, 2, 3, 4, 5]
newReversed; // [5, 4, 3, 2, 1]

Array.toSorted()

const original = [1, 4, 2, 5, 3];
const newSorted = original.toSorted(); // Doesn't change the original array

original; // [1, 4, 2, 5, 3]
newSorted; // [1, 2, 3, 4, 5]

Array.toSpliced()

const original = [1, 3, 4, 5];
const newSpliced = original.toSpliced(1, 0, 2); // Doesn't change the original array

original; // [1, 3, 4, 5]
newSpliced; // [1, 2, 3, 4, 5]

Array.with()

const original = [1, 3, 3, 4, 5];
const newCorrected = original.with(1, 2); // Doesn't change the original array

original; // [1, 3, 3, 4, 5]
newCorrected; // [1, 2, 3, 4, 5]

toReversed, toSorted, and with will also be added to TypedArrays.

Array find from last index

Adds new methods to Array.prototype and TypedArray.prototype: findLast() and findLastIndex() to search an array starting from the end instead of the beginning, that means in the reverse order of find() and findIndex(). It can be useful when you know that finding an element from last to first will have a better performance or, in case the array is sorted and you care about the order of elements.

So it solves the issues that the workaround [].reverse().find() has:

• unnecessary mutation (by reverse).
• unnecessary copy (to avoid mutation)

const isValueOdd = (obj) => obj.value % 2 === 1;
const values = [{ value: 1 }, { value: 2 }, { value: 3 }, { value: 4 }];

values.findLast(isValueOdd); // { value: 3 }
values.findLastIndex(isValueOdd); // 2

Symbols as WeakMap keys

This allows using unique Symbols as keys of a WeakMap that were previously limited to using Objects as keys.

const weakMap = new WeakMap();
const key = Symbol('ref');

weakMap.set(key, 'value');
weakMap.get(key); // 'value'

Hashbang Grammar (#!)

Hashbang or Shebang comment #! is used at the beginning of a executable script to specify the interpreter for the script to be run on. This standardizes how JavaScript engines handle these comments.

#!/usr/bin/env node

console.log('Hi');