What are immutable array operations and the new ES2023 methods?

Many array methods mutate the original. Always be aware of which do and which don't:

Mutating (change original): sort, reverse, splice, push, pop, shift, unshift, fill

Non-mutating (return new): map, filter, slice, concat, flat, flatMap, reduce

const arr = [3, 1, 2];

// Old pattern — copy first to avoid mutation
const sorted = [...arr].sort((a, b) => a - b); // arr unchanged

// ES2023 — built-in immutable versions
arr.toSorted((a, b) => a - b);  // [1, 2, 3] — arr still [3, 1, 2]
arr.toReversed();               // [2, 1, 3] — arr still [3, 1, 2]
arr.toSpliced(1, 1, 9);         // [3, 9, 2] — arr still [3, 1, 2]
arr.with(1, 99);                // [3, 99, 2] — replace index 1

// All 4 return NEW arrays without touching the original
console.log(arr); // [3, 1, 2] ✓